This is a partial list of recent publications and preprints by members of the group. More can be found by searching in the quant-ph arXiv.

• Spectra and eigenstates of spin chain Hamiltonians
J. P. Keating, N. Linden, H. J. Wells
06 March 2014

Abstract:
We prove that translationally invariant Hamiltonians of $n$ qubits with nearest-neighbour interactions have two seemingly contradictory features. Firstly (in the limit $n\rightarrow\infty$) that almost all eigenstates have maximal entanglement between fixed-size sub-blocks of qubits and the rest of the system; in this sense these eigenstates are like those of completely general Hamiltonians (i.e. Hamiltonians with interactions of all orders between arbitrary groups of qubits). Secondly we will show that the density of states of such systems is Gaussian; thus as far as the eigenvalues are concerned the system is like a non-interacting one. The results apply to chains of qubits with translation invariant nearest-neighbour interactions, but we will show that many of the results are extendible to much more general systems (both in terms of the local dimension, the geometry of interaction and, in the case of the density of states, the requirement of translation invariance).
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• Random matrices and quantum spin chains
J. P. Keating, N. Linden, H. J. Wells
06 March 2014

Abstract:
Random matrix ensembles are introduced that respect the local tensor structure of Hamiltonians describing a chain of $n$ distinguishable spin-half particles with nearest-neighbour interactions. We prove a central limit theorem for the density of states when $n \rightarrow\infty$, giving explicit bounds on the rate of approach to the limit. Universality within a class of probability measures and the extension to more general interaction geometries are established. The level spacing distributions of the Gaussian Orthogonal, Unitary and Symplectic Ensembles are observed numerically for the energy levels in these ensembles.
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• Limitations on Quantum Key Repeaters
Stefan Bäuml, Matthias Christandl, Karol Horodecki, Andreas Winter
25 February 2014

Abstract:
A main application of quantum communication is the distribution of entangled particles for use in quantum key distribution (QKD). Due to unavoidable noise in the communication line, QKD is in practice limited to a distance of a few hundred kilometers and can only be extended to longer distances by use of a future quantum repeater, a small-scale quantum computer which performs iterated entanglement distillation and quantum teleportation. The existence of entangled particles that are undistillable but nevertheless useful for QKD raises the question for a quantum key repeater which works beyond the limits of entanglement distillation. In this work we show that any such apparatus is severely limited in its performance; in particular, we exhibit entanglement suitable for QKD but unsuitable for the most general quantum key repeater protocol. The mathematical techniques we develop can be viewed as a step towards opening the theory of entanglement measures to networks of communicating parties.
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• Interfacing Superconducting Qubits and Telecom Photons via a Rare-Earth Doped Crystal
Christopher O'Brien, Nikolai Lauk, Susanne Blum, Giovanna Morigi, Michael Fleischhauer
24 February 2014

Abstract:
We propose a scheme to couple short single photon pulses to superconducting qubits. An optical photon is first absorbed into an inhomogeneously broadened rare-earth doped crystal using controlled reversible inhomogeneous broadening. The optical excitation is then mapped into a spin state using a series of $\pi$-pulses and subsequently transferred to a superconducting qubit via a microwave cavity. To overcome the intrinsic and engineered inhomogeneous broadening of the optical and spin transitions in rare earth doped crystals, we make use of a special transfer protocol using staggered $\pi$-pulses. We predict total transfer efficiencies on the order of 90%.
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• On properties of Karlsson Hadamards and sets of Mutually Unbiased Bases in dimension six
Andrew Maxwell, Stephen Brierley
18 February 2014

Abstract:
The complete classification of all 6x6 complex Hadamard matrices is an open problem. The 3-parameter Karlsson family encapsulates all Hadamards that have been parametrised explicitly. We prove that such matrices satisfy a non-trivial constraint conjectured to hold for (almost) all 6x6 Hadamard matrices. Our result imposes additional conditions in the linear programming approach to the mutually unbiased bases problem recently proposed by Matolcsi et al. Unfortunately running the linear programs we were unable to conclude that a complete set of mutually unbiased bases cannot be constructed from Karlsson Hadamards alone.
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• Strong converse for the quantum capacity of the erasure channel for almost all codes
Mark M. Wilde, Andreas Winter
18 February 2014

Abstract:
A strong converse theorem for channel capacity establishes that the error probability in any communication scheme for a given channel necessarily tends to one if the rate of communication exceeds the channel's capacity. Establishing such a theorem for the quantum capacity of degradable channels has been an elusive task, with the strongest progress so far being a so-called "pretty strong converse". In this work, Morgan and Winter proved that the quantum error of any quantum communication scheme for a given degradable channel converges to a value larger than $1/\sqrt{2}$ in the limit of many channel uses if the quantum rate of communication exceeds the channel's quantum capacity. The present paper establishes a theorem that is a counterpart to this "pretty strong converse". We prove that the large fraction of codes having a rate exceeding the erasure channel's quantum capacity have a quantum error tending to one in the limit of many channel uses. Thus, our work adds to the body of evidence that a fully strong converse theorem should hold for the quantum capacity of the erasure channel. As a side result, we prove that the classical capacity of the quantum erasure channel obeys the strong converse property.
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• One-way Einstein-Podolsky-Rosen steering
Joseph Bowles, Tamás Vértesi, Marco Túlio Quintino, Nicolas Brunner
18 February 2014

Abstract:
Einstein-Podolsky-Rosen steering is a form of quantum nonlocality exhibiting an inherent asymmetry between the observers, Alice and Bob. We present a simple class of entangled two-qubit states which are one-way steerable, considering arbitrary projective measurements. That is, Alice can steer the state of Bob, but it is impossible for Bob to steer Alice's state. This shows that the nonlocal properties of entangled states can be fundamentally asymmetrical.
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• Quantum-enhanced tomography of unitary processes
Xiao-Qi Zhou, Hugo Cable, Rebecca Whittaker, Peter Shadbolt, Jeremy L. O'Brien, Jonathan C.F. Matthews
13 February 2014

Abstract:
A fundamental task in photonics is to characterise an unknown optical process, defined by properties such as birefringence, spectral response, thickness and flatness. Amongst many ways to achieve this, single-photon probes can be used in a method called quantum process tomography (QPT). Furthermore, QPT is an essential method in determining how a process acts on quantum mechanical states. For example for quantum technology, QPT is used to characterise multi-qubit processors and quantum communication channels; across quantum physics QPT of some form is often the first experimental investigation of a new physical process, as shown in the recent research into coherent transport in biological mechanisms. However, the precision of QPT is limited by the fact that measurements with single-particle probes are subject to unavoidable shot noise---this holds for both single photon and laser probes. In situations where measurement resources are limited, for example, where the process is rapidly changing or the time bandwidth is constrained, it becomes essential to overcome this precision limit. Here we devise and demonstrate a scheme for tomography which exploits non-classical input states and quantum interferences; unlike previous QPT methods our scheme capitalises upon the possibility to use simultaneously multiple photons per mode. The efficiency---quantified by precision per photon used---scales with larger photon number input states. Our demonstration uses four-photon states and our results show a substantial reduction of statistical fluctuations compared to traditional QPT methods---in the ideal case one four-photon probe state yields the same amount of statistical information as twelve single probe photons.
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• Closed sets of correlations: answers from the zoo
Ben Lang, Tamas Vertesi, Miguel Navascues
13 February 2014

Abstract:
We investigate the conditions under which a set of multipartite nonlocal correlations can describe the distributions achievable by distant parties conducting experiments in a consistent universe. Several questions are posed, such as: are all such sets "nested", i.e., contained into one another? Are they discrete or do they form a continuum? How many of them are supraquantum? Are there non-trivial polytopes among them? We answer some of these questions or relate them with established conjectures in complexity theory by introducing a "zoo" of physically consistent sets which can be characterized efficiently via either linear or semidefinite programming. As a bonus, we use the zoo to derive, for the first time, concrete impossibility results in nonlocality distillation.
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• The frequency crossover for the Goos-Hanchen shift
Manoel Araujo, Silvania Carvalho, Stefano De Leo
12 February 2014

Abstract:
For total reflection, the Goos-Hanchen (GH) shift is proportional to the wavelength of the laser beam. At critical angles, such a shift is instead proportional to the square root of the product of the beam waist and wavelength. By using the stationary phase method (SPM) and, when necessary, numerical calculations, we present a detailed analysis of the frequency crossover for the GH shift. The study, done in different incidence regions, sheds new light on the validity of the analytic formulas found in literature.
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• Interface between path and OAM entanglement for high-dimensional photonic quantum information
Robert Fickler, Radek Lapkiewicz, Marcus Huber, Martin P. J. Lavery, Miles J. Padgett, Anton Zeilinger
12 February 2014

Abstract:
Photonics has become a mature field of quantum information science, where integrated optical circuits offer a way to scale the complexity of the setup as well as the dimensionality of the quantum state. On photonic chips, paths are the natural way to encode information. To distribute those high-dimensional quantum states over large distances, transverse spatial modes, like orbital angular momentum (OAM) possessing Laguerre Gauss modes, are favourable as flying information carriers. Here we demonstrate a quantum interface between these two vibrant photonic fields. We create three-dimensional path entanglement between two photons in a non-linear crystal and use a mode sorter as the quantum interface to transfer the entanglement to the OAM degree of freedom. Thus our results show a novel, flexible way to create high-dimensional spatial mode entanglement. Moreover, they pave the way to implement broad complex quantum networks where high-dimensionally entangled states could be distributed over distant photonic chips.
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• Immanants for Three-Channel Linear Optical Networks
Hubert de Guise, Si-Hui Tan, Isaac P. Poulin, Barry C. Sanders
12 February 2014

Abstract:
We use permutation-group methods plus SU(3) group-theoretic methods to determine the action of a three-channel passive optical interferometer on single-photon pulse inputs to each channel. Our mathematical description connects partial distinguishability of input photons with linear superpositions of immanants of the interferometer SU(3) matrix. By delaying identical photons, partial distinguishability is controllable, and landscapes of coincidence rates vs delay times between pairs of photons are explained in terms of immanants.
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• Device and semi-device independent random numbers based on non-inequality paradox
Hong-Wei Li, Marcin Pawlowski, Ramij Rahaman, Guang-Can Guo, Zheng-Fu Han
11 February 2014

Abstract:
In this work, we propose device independent true random numbers generation protocols based on non-inequality paradoxes such as Hardy's and Cabello's non-locality argument. The efficiency of generating randomness in our protocols are far better than any other proposed protocols certified by CHSH inequality or other non-locality test involving inequalities. Thus, highlighting non-inequality paradox as an important resource for device independent quantum information processing in particular generating true randomness. As a byproduct, we find that the non-local bound of the Cabello's argument with arbitrary dimension is the same as the one achieved in the qubits system. More interestingly, we propose a new dimension witness paradox based on the Cabello's argument, which can be used for constructing semi-device-independent true random numbers generation protocol.
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• Quantum Systems Equilibrate Rapidly for Most Observables
Artur S.L. Malabarba, Luis Pedro García-Pintos, Noah Linden, Terence C. Farrelly, Anthony J. Short
06 February 2014

Abstract:
Given any Hamiltonian and any initial state spread over many discrete energy levels, we show that most observables are already equlibrated, and that most observables with a definite initial value (which are typically initially out of equilibrium) equilibrate fast. Moreover, all two-outcome measurements, where one of the projectors is of low rank, equilibrate fast.
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• Device-independent randomness extraction for arbitrarily weak min-entropy source
Jan Bouda, Marcin Pawlowski, Matej Pivoluska, Martin Plesch
06 February 2014

Abstract:
In this paper we show how to extract a single random bit with an arbitrarily low bias using a single and arbitrarily weak min-entropy source ($(n,2)$ source for an arbitrary $n$), in a device independent setting. To do this we need the number of devices polynomial in $n$. Our solution is robust, it works with devices that malfunction with probability dropping polynomially in $n$.
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• Intrinsic asymmetry with respect to adversary: new feature of Bell inequalities
P. Horodecki, M. Pawlowski, R. Horodecki
06 February 2014

Abstract:
It is known that the local bound of a Bell inequality is sensitive to the knowledge of the external observer about the settings statistics. Here we ask how that sensitivity depends on the structure of that knowledge. It turns out that in some cases it may happen that the local bound is much more sensitive to adversary's knowledge about settings of one party than the other. Remarkably, there are Bell inequalities which are highly asymmetric with respect to the adversary's knowledge about local settings. This property may be viewed as a hidden intrinsic asymmetry of Bell inequalities. Potential implications of the revealed asymmetry effect are also discussed.
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• Upper bounds on error probabilities and asymptotic error exponents in quantum multiple state discrimination
Koenraad M.R. Audenaert, Milán Mosonyi
30 January 2014

Abstract:
We consider the multiple hypothesis testing problem for symmetric quantum state discrimination between r given states \sigma_1,...,\sigma_r. By splitting up the overall test into multiple binary tests in various ways we obtain a number of upper bounds on the optimal error probability in terms of the binary error probabilities. These upper bounds allow us to deduce various bounds on the asymptotic error rate, for which it has been hypothesised that it is given by the multi-hypothesis quantum Chernoff bound (or Chernoff divergence) C(\sigma_1,...,\sigma_r), as recently introduced by Nussbaum and Szko{\l}a in analogy with Salikhov's classical multi-hypothesis Chernoff bound. This quantity is defined as the minimum of the pairwise binary Chernoff divergences min_{j<k}C(\sigma_j,\sigma_k). It was known already that the optimal asymptotic rate must lie between C/3 and C, and that for certain classes of sets of states the bound is actually achieved. It was known to be achieved, in particular, when the state pair that is closest together in Chernoff divergence is more than 6 times closer than the next closest pair. Our results improve on this in two ways. Firstly, we show that the optimal asymptotic rate must lie between C/2 and C. Secondly, we show that the Chernoff bound is already achieved when the closest state pair is more than 2 times closer than the next closest pair. We also show that the Chernoff bound is achieved when at least $r-2$ of the states are pure, improving on a previous result by Nussbaum and Szko{\l}a. Finally, we indicate a number of potential pathways along which a proof (or disproof) may eventually be found that the multi-hypothesis quantum Chernoff bound is always achieved.
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• Effect of one-, two-, and three-body atom loss processes on superpositions of phase states in Bose-Josephson junctions
Dominique Spehner, Krzysztof Pawlowski, Giulia Ferrini, Anna Minguzzi
29 January 2014

Abstract:
In a two-mode Bose-Josephson junction formed by a binary mixture of ultracold atoms, macroscopic superpositions of phase states are produced during the time evolution after a sudden quench to zero of the coupling amplitude. Using quantum trajectories and an exact diagonalization of the master equation, we study the effect of one-, two-, and three-body atom losses on the superpositions by analyzing separately the amount of quantum correlations in each subspace with fixed atom number. The quantum correlations useful for atom interferometry are estimated using the quantum Fisher information. We identify the choice of parameters leading to the largest Fisher information, thereby showing that, for all kinds of loss processes, quantum correlations can be partially protected from decoherence when the losses are strongly asymmetric in the two modes.
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• Graph-Theoretic Approach to Quantum Correlations
Adan Cabello, Simone Severini, Andreas Winter
29 January 2014

Abstract:
Correlations in Bell and noncontextuality inequalities can be expressed as a positive linear combination of probabilities of events. Exclusive events can be represented as adjacent vertices of a graph, so correlations can be associated to a subgraph. We show that the maximum value of the correlations for classical, quantum, and more general theories is the independence number, the Lov\'asz number, and the fractional packing number of this subgraph, respectively. We also show that, for any graph, there is always a correlation experiment such that the set of quantum probabilities is exactly the Gr\"otschel-Lov\'asz-Schrijver theta body. This identifies these combinatorial notions as fundamental physical objects and provides a method for singling out experiments with quantum correlations on demand.
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• Dimension of physical systems, information processing, and thermodynamics
Nicolas Brunner, Marc Kaplan, Anthony Leverrier, Paul Skrzypczyk
21 January 2014

Abstract:
We investigate the problem of giving a model independent definition of the dimension of physical systems. We give two definitions of dimension, based on measurements and on the capacity of storing information. While both definitions coincide in classical and quantum mechanics, they are in general different in generalized probabilistic theories. We discuss in detail the case of a theory known as 'boxworld', and show that such a theory features systems with a dimension mismatch. This dimension mismatch can be made arbitrarily large by using an amplification procedure. Furthermore, we show that the dimension mismatch of this model has strong consequences on its power for performing information-theoretic tasks, leading to the collapse of communication complexity and to the violation of information causality. Finally, we discuss the consequences of a dimension mismatch from the perspective of thermodynamics, and ask whether this effect could break Landauer's erasure principle and thus the second law.
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• Black hole qubit correspondence from quantum circuits
Thiago Prudencio, Diego J. Cirilo-Lombardo, Edilberto O. Silva, Humberto Belich
20 January 2014

Abstract:
We propose a black hole qubit correspondence (BHQC) from quantum circuits, taking into account the BHQC formulations of wrapped brane qubits. With base on BHQC, we implement the corresponding gate operations to realize any given quantum circuit. In particular, we implement cases of the generation of Bell states, quantum teleportation and GHZ states circuits. Finally, we give an interpretation of the BHQC from quantum circuits with base on the BHQC classification of entanglement classes.
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• Strong converse for the classical capacity of all phase-insensitive bosonic Gaussian channels
Bhaskar Roy Bardhan, Raul Garcia-Patron, Mark M. Wilde, Andreas Winter
17 January 2014

Abstract:
We prove that a strong converse theorem holds for the classical capacity of all phase-insensitive bosonic Gaussian channels, when imposing a maximum photon number constraint on the inputs of the channel. The pure-loss, thermal, additive noise, and amplifier channels are all in this class of channels. The statement of the strong converse theorem is that the probability of correctly decoding a classical message rapidly converges to zero in the limit of many channel uses if the communication rate exceeds the classical capacity. We prove this theorem by relating the success probability of any code with its rate of data transmission, the effective dimension of the channel output space, and the purity of the channel as quantified by the minimum output entropy. Our result bolsters the understanding of the classical capacity of these channels by establishing it as a sharp dividing line between possible and impossible communication rates over them.
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• Seeing bulk topological properties of band insulators in small photonic lattices
C.-E. Bardyn, S. D. Huber, O. Zilberberg
30 December 2013

Abstract:
We present a general scheme for measuring the bulk properties of non-interacting tight-binding models realized in arrays of coupled photonic cavities. Specifically, we propose to implement a single unit cell of the targeted model with tunable twisted boundary conditions in order to simulate large systems and, most importantly, to access bulk topological properties experimentally. We illustrate our method by demonstrating how to measure topological invariants in a two-dimensional quantum Hall-like model.
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• The coherence of superradiance
Raphael Lopes, Almazbek Imanaliev, Marie Bonneau, Josselin Ruaudel, Marc Cheneau, Denis Boiron, Christoph I Westbrook
27 December 2013

Abstract:
We have measured the 2nd order coherence, or 2-body correlations, of atoms from a Bose--Einstein condensate participating in a superradiance process. We compare the statistics of the superradiant phenomenon with the ordinary spontaneous emission and with a coherent source obtained via a stimulated Raman transition of a Bose--Einstein condensate. Despite strong stimulated emission the correlation properties of the superradiance are close to those of a thermal sample.
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• The coherence of superradiance
Raphael Lopes, Almazbek Imanaliev, Marie Bonneau, Josselin Ruaudel, Marc Cheneau, Denis Boiron, Christoph I Westbrook
26 December 2013

Abstract:
We have measured the 2nd order coherence, or 2-body correlations, of atoms from a Bose--Einstein condensate participating in a superradiance process. We compare the statistics of the superradiant phenomenon with the ordinary spontaneous emission and with a coherent source obtained via a stimulated Raman transition of a Bose--Einstein condensate. Despite strong stimulated emission the correlation properties of the superradiance are close to those of a thermal sample.
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• The coherence of superradiance
Raphael Lopes, Almazbek Imanaliev, Marie Bonneau, Josselin Ruaudel, Marc Cheneau, Denis Boiron, Christoph I Westbrook
25 December 2013

Abstract:
We have measured the 2nd order coherence, or 2-body correlations, of atoms from a Bose--Einstein condensate participating in a superradiance process. We compare the statistics of the superradiant phenomenon with the ordinary spontaneous emission and with a coherent source obtained via a stimulated Raman transition of a Bose--Einstein condensate. Despite strong stimulated emission the correlation properties of the superradiance are close to those of a thermal sample.
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• Auxiliary master equation approach to non-equilibrium correlated impurities
Antonius Dorda, Martin Nuss, Wolfgang von der Linden, Enrico Arrigoni
19 December 2013

Abstract:
We present a numerical method for the study of correlated quantum impurity problems out of equilibrium, which is particularly suited to address steady state properties within Dynamical Mean Field Theory. The approach, recently introduced in [Arrigoni et al., Phys. Rev. Lett. 110, 086403 (2013)], is based upon a mapping of the original impurity problem onto an auxiliary open quantum system, consisting of the interacting impurity coupled to bath sites as well as to a Markovian environment. The dynamics of the auxiliary system is governed by a Lindblad master equation whose parameters are used to optimize the mapping. The accuracy of the results can be readily estimated and systematically improved by increasing the number of auxiliary bath sites, or by introducing a linear correction. Here, we focus on a detailed discussion of the proposed approach including technical remarks. To solve for the Green's functions of the auxiliary impurity problem, a non-hermitian Lanczos diagonalization is applied. As a benchmark, results for the steady state current-voltage characteristics of the single impurity Anderson model are presented. Furthermore, the bias dependence of the single particle spectral function and the splitting of the Kondo resonance are discussed. In its present form the method is fast, efficient and features a controlled accuracy.
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• Measurement of sub-pulse-width temporal delays via spectral interference induced by weak value amplification
Luis Jose Salazar-Serrano, Davide Janner, Nicolas Brunner, Valerio Pruneri, Juan P. Torres
17 December 2013

Abstract:
We demonstrate experimentally a scheme to measure small temporal delays, much smaller than the pulse width, between optical pulses. Specifically, we observe an interference effect, based on the concepts of quantum weak measurements and weak value amplification, through which a sub-pulsewidth temporal delay between two femtosecond pulses induces a measurable shift of the central frequency of the pulse. The amount of frequency shift, and the accompanying losses of the measurement, can be tailored by post-selecting different states of polarization. Our scheme requires only spectrum measurements and linear optics elements, hence greatly facilitating its implementation. Thus it appears as a promising technique for measuring small and rapidly varying temporal delays.
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• Discrete Spacetime and Relativistic Quantum Particles
Terence C. Farrelly, Anthony J. Short
11 December 2013

Abstract:
We study a single quantum particle in discrete spacetime evolving in a causal way. We see that in the continuum limit any massless particle with a two dimensional internal degree of freedom obeys the Weyl equation, provided that we perform a simple relabeling of the coordinate axes or demand rotational symmetry in the continuum limit. It is surprising that this occurs regardless of the specific details of the evolution: it would be natural to assume that discrete evolutions giving rise to relativistic dynamics in the continuum limit would be very special cases. We also see that the same is not true for particles with larger internal degrees of freedom, by looking at an example with a three dimensional internal degree of freedom that is not relativistic in the continuum limit. In the process we give a formula for the Hamiltonian arising from the continuum limit of massless and massive particles in discrete spacetime.
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• Testing randomness with photons
Jonathan C. F. Matthews, Rebecca Whittaker, Jeremy L. O'Brien, Peter S. Turner
09 December 2013

Abstract:
Generating and characterising randomness is fundamentally important in both classical and quantum information science. Here we report the experimental demonstration of ensembles of pseudorandom optical processes comprising what are known as t-designs. We show that in practical scenarios, certain finite ensembles of two-mode transformations---1- and 2-designs---are indistinguishable from truly random operations for 1- and 2-photon quantum interference, but they fail to mimic randomness for 2- and 3-photon cases, respectively. This provides a novel optical test of pseudo randomness. We make use of the fact that t-photon behaviour is governed by degree-2t polynomials in the parameters of the optical process to experimentally verify the ensembles' behaviour for complete bases of polynomials. This ensures that outputs will be uniform for arbitrary configurations, satisfying the strict definition of pseudorandomness implicit in the mathematical definition.
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• Quantifying quantum steering
Paul Skrzypczyk, Miguel Navascues, Daniel Cavalcanti
20 November 2013

Abstract:
Quantum steering is a form of bipartite quantum correlations that is intermediate between entanglement and Bell nonlocality. It allows for entanglement certification when the measurements performed by one of the parties are not characterised (or untrusted) and has applications in quantum key distribution. Despite its foundational and applied importance, quantum steering lacks a quantitative assessment. Here we propose a way of quantifying this phenomenon and study the steering power of several quantum states. In particular we show that every pure entangled state is maximally steerable. Furthermore we study the steering power of several interesting states, and give strong support that states with positive-partial-transposition are not steerable.
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• Device-dependent and device-independent quantum key distribution without a shared reference frame
Joshua A. Slater, Cyril Branciard, Nicolas Brunner, Wolfgang Tittel
15 November 2013

Abstract:
Standard quantum key distribution (QKD) protocols typically assume that the distant parties share a common reference frame. In practice, however, establishing and maintaining a good alignment between distant observers is rarely a trivial issue, which may significantly restrain the implementation of long-distance quantum communication protocols. Here we propose simple QKD protocols that do not require the parties to share any reference frame, and study their security and feasibility in both the usual device-dependent case--in which the two parties use well characterized measurement devices--as well as in the device-independent case--in which the measurement devices can be untrusted, and the security relies on the violation of a Bell inequality. To illustrate the practical relevance of these ideas, we present a proof-of-principle demonstration of our protocols using polarization entangled photons distributed over a coiled 10-km-long optical fiber. We consider two situations, in which either the fiber spool freely drifts, or randomly chosen polarization transformations are applied. The correlations obtained from measurements allow, with high probability, to generate positive asymptotic secret key rates in both the device-dependent and device-independent scenarios (under the fair-sampling assumption for the latter case).
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• Complexity classification of local Hamiltonian problems
Toby Cubitt, Ashley Montanaro
14 November 2013

Abstract:
The calculation of ground-state energies of physical systems can be formalised as the k-local Hamiltonian problem, which is the natural quantum analogue of classical constraint satisfaction problems. One way of making the problem more physically meaningful is to restrict the Hamiltonian in question by picking its terms from a fixed set S. Examples of such special cases are the Heisenberg and Ising models from condensed-matter physics.
In this work we characterise the complexity of this problem for all 2-local qubit Hamiltonians. Depending on the subset S, the problem falls into one of the following categories: in P; NP-complete; polynomial-time equivalent to the Ising model with transverse magnetic fields; or QMA-complete. The third of these classes contains NP and is contained within StoqMA. The characterisation holds even if S does not contain any 1-local terms; for example, we prove for the first time QMA-completeness of the Heisenberg and XY interactions in this setting. If S is assumed to contain all 1-local terms, which is the setting considered by previous work, we have a characterisation that goes beyond 2-local interactions: for any constant k, all k-local qubit Hamiltonians whose terms are picked from a fixed set S correspond to problems either in P; polynomial-time equivalent to the Ising model with transverse magnetic fields; or QMA-complete.
These results are a quantum analogue of Schaefer's dichotomy theorem for boolean constraint satisfaction problems.
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• On the experimental verification of quantum complexity in linear optics
Jacques Carolan, Pete Shadbolt, Jasmin D. A. Meinecke, Nicholas J. Russell, Nur Ismail, Kerstin Wörhoff, Terry Rudolph, Mark G. Thompson, Jeremy L. O'Brien, Jonathan C. F. Matthews, Anthony Laing
13 November 2013

Abstract:
The first quantum technologies to solve computational problems that are beyond the capabilities of classical computers are likely to be devices that exploit characteristics inherent to a particular physical system, to tackle a bespoke problem suited to those characteristics. Evidence exists to imply that the detection of ensembles of photons, which have propagated through a linear optical circuit, is equivalent to sampling from a probability distribution that is intractable to classical simulation. However, it is probable that the complexity of this type of sampling problem means that its solution is classically unverifiable and the task of establishing correct operation becomes one of gathering sufficiently convincing circumstantial evidence. Here, we develop scalable methods to experimentally establish correct operation for this class of sampling algorithm, which we test with different types of optical circuits for 3, 4, and 5 photons, on Hilbert spaces of up to 50k dimensions. With only a small number of trials, we are > 99% confident that we are not sampling from a uniform or classical distribution, and we demonstrate a unitary specific witness that functions robustly for small amounts of data. Like the algorithmic operations they seek to endorse, our approach is to develop methods that exploit the characteristics native to the quantum system in question. Here we observe and make an application of a bosonic clouding phenomenon, where photons are found in local groups of modes superposed across two locations, which is interesting in its own right as a basic multi-particle phenomenon. Our broad approach is likely to be practical for all architectures for quantum technologies where formal verification methods for native algorithms are either intractable or unknown.
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• Certifying the dimension of classical and quantum systems in a prepare-and-measure scenario with independent devices
Joseph Bowles, Marco Tulio Quintino, Nicolas Brunner
08 November 2013

Abstract:
We consider the problem of testing the dimension of uncharacterised classical and quantum systems in a prepare-and-measure setup. Here we assume the preparation and measurement devices to be independent, thereby making the problem non-convex. We present a simple method for generating nonlinear dimension witnesses for systems of arbitrary dimension. The simplest of our witnesses is highly robust to technical imperfections, and can certify the use of qubits in the presence of arbitrary noise and arbitrarily low detection efficiency. Finally, we show that this witness can be used to certify the presence of randomness, suggesting applications in quantum information processing.
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• Renyi divergences and the classical capacity of finite compound channels
Milan Mosonyi
29 October 2013

Abstract:
We show two-sided bounds between the conventional quantum R\'enyi divergences and the new notion of R\'enyi divergences introduced recently in M\"uller-Lennert, Dupuis, Szehr, Fehr and Tomamichel, arXiv:1306.3142, and Wilde, Winter, Yang, arXiv:1306.1586. The bounds imply that the two versions can be used interchangably near alpha=1, and hence one can benefit from the best properties of both when proving coding theorems in the case of asymptotically vanishing error. We illustrate this by giving a short and simple proof for the achievability of the capacity formula given by Datta and Dorlas for averaged channels with finitely many i.i.d. branches, which yields the same lower bound for the classical capacity of finite compound channels. The proof is based on very general arguments; the main ingredients are Hayashi and Nagaoka's single-shot random coding theorem, and a weak quasi-concavity property of the Renyi divergences that we also establish here.
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• Bounds on entanglement assisted source-channel coding via the Lovasz theta number and its variants
Toby Cubitt, Laura Mancinska, David Roberson, Simone Severini, Dan Stahlke, Andreas Winter
29 October 2013

Abstract:
The entanglement assisted independence number is a graph theoretic quantity introduced in the context of zero-error channel capacity, and is equal to the number of codewords that can be transmitted error-free through a classical channel with the help of entanglement. Beigi introduced a quantity $\beta$ as an upper bound on the entanglement assisted independence number. We adapt and extend Beigi's argument to the context of source-channel coding (communication in the presence of side information). Entanglement assisted source-channel coding is possible only if there exists a set of vectors satisfying certain orthogonality conditions related to suitably defined graphs $G$ and $H$. We show that such vectors exist if and only if $\vartheta(\overline{G}) \le \vartheta(\overline{H})$ where $\vartheta$ represents the Lov\'asz number. We also obtain similar results for the related Schrijver $\vartheta^-$ and Szegedy $\vartheta^+$ numbers.
These inequalities reproduce or generalize several known results, provide a bound on entanglement assisted source-channel coding, and provide tightened bounds on entanglement assisted one-shot zero-error capacity. In particular, we show that the entanglement assisted independence number is bounded by the Schrijver number: $\alpha^*(G) \le \vartheta^-(G)$. Therefore, we are able to disprove the conjecture that the one-shot entanglement-assisted zero-error capacity is equal to the integer part of the Lov\'asz number. Finally, Beigi posed the question of whether $\beta = \lfloor \vartheta \rfloor$. We answer this in the affirmative and show that a related quantity is equal to $\lceil \vartheta \rceil$.
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• A Survey of Quantum Property Testing
Ashley Montanaro, Ronald de Wolf
09 October 2013

Abstract:
The area of property testing tries to design algorithms that can efficiently handle very large amounts of data: given a large object that either has a certain property or is somehow "far" from having that property, a tester should efficiently distinguish between these two cases. In this survey we describe recent results obtained for quantum property testing. This area naturally falls into three parts. First, we may consider quantum testers for properties of classical objects. We survey the main examples known where quantum testers can be much (sometimes exponentially) more efficient than classical testers. Second, we may consider classical testers of quantum objects. This is the situation that arises for instance when one is trying to determine if quantum states or operations do what they are supposed to do, based only on classical input-output behavior. Finally, we may also consider quantum testers for properties of quantum objects, such as states or operations. We survey known bounds on testing various natural properties, such as whether two states are equal, whether a state is separable, whether two operations commute, etc. We also highlight connections to other areas of quantum information theory and mention a number of open questions.
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• Genuinely multi-point temporal quantum correlations
Marcin Markiewicz, Anna Przysiezna, Stephen Brierley, Tomasz Paterek
01 October 2013

Abstract:
Due to a restrictive scenario of sequential projective measurements, multi-point temporal quantum correlations were thought to factorize into two-point ones. We show that sequential generalized measurements lead to genuinely multi-point temporal correlations, which enable us to translate typical spatial genuinely multipartite nonlocal phenomena, like Greenberger-Horne-Zeilinger paradox, into the temporal scenario.
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• Quantum walks of correlated photon pairs in two-dimensional waveguide arrays
Konstantinos Poulios, Robert Keil, Daniel Fry, Jasmin D. A. Meinecke, Jonathan C. F. Matthews, Alberto Politi, Mirko Lobino, Markus Gräfe, Matthias Heinrich, Stefan Nolte, Alexander Szameit, Jeremy L. O'Brien
13 August 2013

Abstract:
We demonstrate quantum walks of correlated photons in a 2D network of directly laser written waveguides coupled in a 'swiss cross' arrangement. The correlated detection events show high-visibility quantum interference and unique composite behaviour: strong correlation and independence of the quantum walkers, between and within the planes of the cross. Violations of a classically defined inequality, for photons injected in the same plane and in orthogonal planes, reveal non-classical behaviour in a non-planar structure.
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• Multi-Colour Quantum Metrology with Entangled Photons
Bryn Bell, Srikanth Kannan, Alex McMillan, Alex S. Clark, William J. Wadsworth, John G. Rarity
13 August 2013

Abstract:
Entangled photons can be used to make measurements with an accuracy beyond that possible with classical light. While most implementations of quantum metrology have used states made up of a single colour of photons, we show that entangled states of two colours can show supersensitivity to optical phase and path-length by using a photonic crystal fibre source of photon pairs inside an interferometer. This setup is relatively simple and robust to experimental imperfections. We demonstrate sensitivity beyond the standard quantum limit and show super-resolved interference fringes using entangled states of two, four, and six photons.
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• Pre- and post-selected quantum states: density matrices, tomography, and Kraus operators
Ralph Silva, Yelena Guryanova, Nicolas Brunner, Noah Linden, Anthony J. Short, Sandu Popescu
12 August 2013

Abstract:
We present a general formalism for charecterizing 2-time quantum states, describing pre- and post-selected quantum systems. The most general 2-time state is characterized by a density vector' that is independent of measurements performed between the preparation and post-selection. We provide a method for performing tomography of an unknown 2-time density vector. This procedure, which cannot be implemented by weak or projective measurements, brings new insight to the fundamental role played by Kraus operators in quantum measurements. Finally, after showing that general states and measurements are isomorphic, we show that any measurement on a 2-time state can be mapped to a measurement on a preselected bipartite state.
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• Inequalities for the Ranks of Quantum States
Josh Cadney, Marcus Huber, Noah Linden, Andreas Winter
05 August 2013

Abstract:
We investigate relations between the ranks of marginals of multipartite quantum states. These are the Schmidt ranks across all possible bipartitions and constitute a natural quantification of multipartite entanglement dimensionality. We show that there exist inequalities constraining the possible distribution of ranks. This is analogous to the case of von Neumann entropy (\alpha-R\'enyi entropy for \alpha=1), where nontrivial inequalities constraining the distribution of entropies (such as e.g. strong subadditivity) are known. It was also recently discovered that all other \alpha-R\'enyi entropies for $\alpha\in(0,1)\cup(1,\infty)$ satisfy only one trivial linear inequality (non-negativity) and the distribution of entropies for $\alpha\in(0,1)$ is completely unconstrained beyond non-negativity. Our result resolves an important open question by showing that also the case of \alpha=0 (logarithm of the rank) is restricted by nontrivial linear relations and thus the cases of von Neumann entropy (i.e., \alpha=1) and 0-R\'enyi entropy are exceptionally interesting measures of entanglement in the multipartite setting.
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• Practical Quantum Metrology
Jonathan C. F. Matthews, Xiao-Qi Zhou, Peter J. Shadbolt, Hugo Cable, Peter J. Shadbolt, Dylan J. Saunders, Gabriel A. Durkin, Geoff J. Pryde, Jeremy L. O'Brien
18 July 2013

Abstract:
Quantum metrology research promises approaches to build new sensors that achieve the ultimate level of precision measurement and perform fundamentally better than modern sensors. Practical schemes that tolerate realistic fabrication imperfections and environmental noise are required in order to realise quantum-enhanced sensors and to enable their real-world application. We have demonstrated the key enabling principles of a practical, loss-tolerant approach to photonic quantum metrology designed to harness all multi-photon components in spontaneous parametric downconversion---a method for generating multiple photons that we show requires no further fundamental state engineering for use in practical quantum metrology. We observe a quantum advantage of 28% in precision measurement of optical phase using the four-photon detection component of this scheme, despite 83% system loss. This opens the way to new quantum sensors based on current quantum-optical capabilities.
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• Genuine hidden quantum nonlocality
Flavien Hirsch, Marco Túlio Quintino, Joseph Bowles, Nicolas Brunner
18 July 2013

Abstract:
The nonlocality of certain quantum states can be revealed by using local filters before performing a standard Bell test. This phenomenon, known as hidden nonlocality, has been so far demonstrated only for a restricted class of measurements, namely projective measurements. Here we prove the existence of genuine hidden nonlocality. Specifically, we present a class of two-qubit entangled states, for which we construct a local model for the most general local measurements (POVMs), and show that the states violate a Bell inequality after local filtering. Hence there exist entangled states, the nonlocality of which can be revealed only by using a sequence of measurements. Finally, we show that genuine hidden nonlocality can be maximal. There exist entangled states for which a sequence of measurements can lead to maximal violation of a Bell inequality, while the statistics of non-sequential measurements is always local.
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• The entropy vector formalism and the structure of multidimensional entanglement in multipartite systems
Marcus Huber, Martí Perarnau-Llobet, Julio I. de Vicente
15 July 2013

Abstract:
We review and generalize the recently introduced framework of entropy vectors for detecting and quantifying genuine multipartite entanglement in high dimensional multicomponent quantum systems. We show that these ideas can be extended to discriminate among other forms of multipartite entanglement. In particular, we develop methods to test whether density matrices are: decomposable, i.e. separable with respect to certain given partitions of the subsystems; k-separable, i.e. separable with respect to k-partitions of the subsystems; k-partite entangled, i.e. there is no entanglement among subsets of more than k parties. We also discuss how to asses the dimensionality of entanglement in all these cases.
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• Two-photon interference between disparate sources for quantum networking
A. R. McMillan, L. Labonté, A. S. Clark, B. Bell, O. Alibart, A. Martin, W. J. Wadsworth, S. Tanzilli, J. G. Rarity
26 June 2013

Abstract:
Quantum networks involve entanglement sharing between multiple users. Ideally, any two users would be able to connect regardless of the type of photon source they employ, provided they fulfill the requirements for two-photon interference. From a theoretical perspective, photons coming from different origins can interfere with a perfect visibility, provided they are made indistinguishable in all degrees of freedom. Previous experimental demonstrations of such a scenario have been limited to photon wavelengths below 900 nm, unsuitable for long distance communication, and suffered from low interference visibility. We report two-photon interference using two disparate heralded single photon sources, which involve different nonlinear effects, operating in the telecom wavelength range. The measured visibility of the two-photon interference is 80+/-4%, which paves the way to hybrid universal quantum networks.
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• Detection-Loophole-Free Test of Quantum Nonlocality, and Applications
B. G. Christensen, K. T. McCusker, J. Altepeter, B. Calkins, T. Gerrits, A. Lita, A. Miller, L. K. Shalm, Y. Zhang, S. W. Nam, N. Brunner, C. C. W. Lim, N. Gisin, P. G. Kwiat
26 June 2013

Abstract:
We present a source of entangled photons that violates a Bell inequality free of the "fair-sampling" assumption, by over 7 standard deviations. This violation is the first experiment with photons to close the detection loophole, and we demonstrate enough "efficiency" overhead to eventually perform a fully loophole-free test of local realism. The entanglement quality is verified by maximally violating additional Bell tests, testing the upper limit of quantum correlations. Finally, we use the source to generate secure private quantum random numbers at rates over 4 orders of magnitude beyond previous experiments.
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• Studies of Quantum Entanglement in 100 Dimensions
Mario Krenn, Marcus Huber, Robert Fickler, Radek Lapkiewicz, Sven Ramelow, Anton Zeilinger
04 June 2013

Abstract:
Entangled quantum systems have properties that have fundamentally overthrown a classical worldview. Increasing the complexity of entangled states by expanding their dimensionality not only allows the implementation of novel fundamental tests of nature, but also enables genuinely new protocols for quantum communication and quantum computation. In our experiment we generate photons entangled in angular momentum and radial modes. We unambiguously verify that these photons are highly entangled in most 2x2-dimensional subspaces of a 34.500-dimensional Hilbert space, which suggests the generation of genuine high dimensional entanglement. We develop a source-independent criterion that reveals an entanglement dimensionality of over 100. For the criterion we propose a mathematical conjecture for which we have strong numerical evidence and theoretical arguments. Furthermore, the size of the entangled Hilbert space is of the same magnitude as the largest entangled multipartite systems experimentally measured so far. This result indicates the great potential of high-dimensional entangled photons for a wide range of quantum information tasks.
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• Entanglement enhances performance in microscopic quantum fridges
Nicolas Brunner, Marcus Huber, Noah Linden, Sandu Popescu, Ralph Silva, Paul Skrzypczyk
28 May 2013

Abstract:
Understanding the thermodynamics of quantum systems is of fundamental importance, from both theoretical and experimental perspectives. A growing interest has been recently given to small self-contained quantum thermal machines, the functioning of which requires no external source of work or control, but only incoherent interactions with thermal baths. The simplicity of such machines makes them and ideal test-bed for exploring quantum thermodynamics. So far, however, the importance of quantum effects in these machines has remained elusive. Here we show that entanglement, the paradigmatical quantum effect, plays a fundamental role in small self-contained quantum refrigerators, as it can enhance cooling and energy transport -- except notably when the efficiency is close to the Carnot limit. Hence a truly quantum refrigerator can outperform a classical one. Furthermore, the amount of entanglement alone quantifies the enhancement in cooling. More generally, our work shows that entanglement opens new possibilities in thermodynamics.
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• Activation of entanglement in teleportation
Ryszard Weinar, Wieslaw Laskowski, Marcin Pawlowski
16 May 2013

Abstract:
We study the activation of entanglement in teleportation protocols. To this end, we a present derivation of the average fidelity of teleportation process with noisy classical channel for qudits. In our work we do not make any assumptions about the entangled states shared by communicating parties. Our result allows us to specify the minimum amount of classical information required to beat the classical limit when the protocol is based on the Bell measurements. We also compare average fidelity of teleportation obtained using noisy and perfect classical channel with restricted capacity. The most important insight into the intricacies of quantum information theory that we gain is that though entanglement, obviously, is a necessary resource for efficient teleportation it requires a certain threshold amount of classical communication to be more useful than classical communication. Another interesting finding is that the amount of classical communication required to activate entanglement for teleportation purposes depends on the dimension d of the system being teleported but is not monotonic reaching maximum for d = 4.
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• Quantum enhanced metrology in the presence of arbitrary loss
J.G. Rarity, B. Bell, A.B. Young, C.Y. Hu
16 May 2013

Abstract:
We introduce the concept of entanglement enhanced interferometry from the viewpoint of the detected photons. The standard quantum limit is achieved when sequentially detected photons are assumed to be in an uncorrelated product state. However when we access the correlations between the detected photons that existed before the interferometer it becomes clear that entanglement enhanced measurement beyond the quantum limit could be achieved independent of loss. We describe possible realisations of this post-measurement entanglement detection using a small array of spin photon entangling gates. We then describe a proof of principle experiment using only linear optics resources.
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• Each instant of time a new Universe
Yakir Aharonov, Sandu Popescu, Jeff Tollaksen
08 May 2013

Abstract:
We present an alternative view of quantum evolution in which each moment of time is viewed as a new "universe" and time evolution is given by correlations between them.
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• Coherently enhanced measurements in classical mechanics
Daniel Braun, Sandu Popescu
07 May 2013

Abstract:
We show that the recently discovered quantum-enhanced measurement protocol of coherent averaging that is capable of achieving Heisenberg-limited sensitivity without using entanglement, has a classical analogue. The classical protocol uses N harmonic oscillators coupled to a central oscillator and one measures the signal from the latter. We propose an application to the measurement of very weak interactions, and, in particular, a novel route to measuring the gravitational constant with enhanced precision.
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• Negative Conditional Entropy of Post-Selected States
Sina Salek, Roman Schubert, Karoline Wiesner
07 May 2013

Abstract:
Using Dirac complex distribution, and hence the statistics of weak measurements, we discuss a decomposition of "conditional state" of post-selected systems and introduce an entropic measure of information for them. In doing so we remark on the role of pre- and post- selection in the measurement of an ensemble. Conditional states are the quantum analogues of the conditional probabilities. We define them by selecting a particular condition in the measurement of a quantum system and studying a coarse grained set of events in the history of the state that ended in that particular condition. These states are different from what is known as conditional states in the literature [16, 6], in the sense that they are trace-1 operators and, by construction, they can be measured using weak measurements. We shall then define a conditional entropic measure based on these states, which as opposed to their classical counterparts, can have negative values. This is also the case even in the case of single state systems. This negative conditional entropy quantifies the amount of information in the post-selected ensembles, states which signify a non-separable class of histories of a quantum system.
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• Experimental characterization of universal one-way quantum computing
B. A. Bell, M. S. Tame, A. S. Clark, R. W. Nock, W. J. Wadsworth, J. G. Rarity
02 May 2013

Abstract:
We report the characterization of a universal set of logic gates for one-way quantum computing using a four-photon star' cluster state generated by fusing photons from two independent photonic crystal fibre sources. We obtain a fidelity for the cluster state of 0.66 +/- 0.01 with respect to the ideal case. We perform quantum process tomography to completely characterize a controlled-NOT, Hadamard and T gate all on the same compact entangled resource. Together, these operations make up a universal set of gates such that arbitrary quantum logic can be efficiently constructed from combinations of them. We find process fidelities with respect to the ideal cases of 0.64 +/- 0.01 for the CNOT, 0.67 +/- 0.03 for the Hadamard and 0.76 +/- 0.04 for the T gate. The characterisation of these gates enables the simulation of larger protocols and algorithms. As a basic example, we simulate a Swap gate consisting of three concatenated CNOT gates. Our work provides some pragmatic insights into the prospects for building up to a fully scalable and fault-tolerant one-way quantum computer with photons in realistic conditions.
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• The classical limit of quantum optics: not what it seems at first sight
Yakir Aharonov, Alonso Botero, Shmuel Nussinov, Sandu Popescu, Jeff Tollaksen, Lev Vaidman
02 May 2013

Abstract:
What is light and how to describe it has always been a central subject in physics. As our understanding has increased, so have our theories changed: Geometrical optics, wave optics and quantum optics are increasingly sophisticated descriptions, each referring to a larger class of phenomena than its predecessor. But how exactly are these theories related? How and when wave optics reduces to geometric optics is a rather simple problem. Similarly, how quantum optics reduces to wave optics has been considered to be a very simple business as well. It's not so. As we show here the classical limit of quantum optics is a far more complicated issue; it is in fact dramatically more involved and it requires a complete revision of all our intuitions. The revised intuitions can then serve as a guide to finding novel quantum effects.
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• Demonstration of Free-space Reference Frame Independent Quantum Key Distribution
J. Wabnig, D. Bitauld, H. W. Li, A. Laing, J. L. O'Brien, A. O. Niskanen
02 May 2013

Abstract:
Quantum key distribution (QKD) is moving from research laboratories towards applications. As computing becomes more mobile, cashless as well as cardless payment solutions are introduced, and a need arises for incorporating QKD in a mobile device. Handheld devices present a particular challenge as the orientation and the phase of a qubit will depend on device motion. This problem is addressed by the reference frame independent (RFI) QKD scheme. The scheme tolerates an unknown phase between logical states that varies slowly compared to the rate of particle repetition. Here we experimentally demonstrate the feasibility of RFI QKD over a free-space link in a prepare and measure scheme using polarisation encoding. We extend the security analysis of the RFI QKD scheme to be able to deal with uncalibrated devices and a finite number of measurements. Together these advances are an important step towards mass production of handheld QKD devices.
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• On-chip quantum interference between two silicon waveguide sources
Joshua Silverstone, Damien Bonneau, Kazuya Ohira, Nob Suzuki, Haruhiko Yoshida, Norio Iizuka, Mizunori Ezaki, Robert Hadfield, Val Zwiller, John Rarity, Jeremy OBrien, Mark Thompson
05 April 2013

Abstract:
Integrated quantum optics promises to enhance the scale and functionality of quantum technologies, and has become a leading platform for the development of complex and stable quantum photonic circuits. Here, we report the on-chip generation and manipulation of two-photon entanglement, and high-visibility quantum interference with two photon-pair sources integrated within a reconfigurable silicon-on-insulator photonic circuit. Degenerate and non-degenerate entangled photon pairs were created and manipulated on-chip to exhibit quantum interference with visibility as high as 100.0 +/- 0.4%. Our device presents integration of photon-pair sources with dynamic quantum photonic circuitry, and the first high-visibility quantum interference between on-chip sources. These results represent a path to the next generation of monolithic quantum photonic circuits with integrated sources.
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• Proposal for a loophole-free Bell test based on spin-photon interactions in cavities
Nicolas Brunner, Andrew B. Young, Chengyong Hu, John G. Rarity
27 March 2013

Abstract:
We present a scheme to demonstrate loophole-free Bell inequality violation where the entanglement between photon pairs is transferred to solid state (spin) qubits mediated by cavity QED interactions. As this transfer can be achieved in a heralded way, our scheme is basically insensitive to losses on the channel. This makes it appealing for the implementation of quantum information protocols based on nonlocality, such as device-independent quantum key distribution. We consider potential experimental realisations of our scheme using single atom, colour centre and quantum dot cavity systems.
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• All entangled states violate a Leggett inequality
Miguel Navascues
22 March 2013

Abstract:
In this note, we prove that any set of bipartite correlations compatible with Leggett's model must necessarily correspond to the statistics generated by a separable two-qubit state. Consequently, any experiment reporting entanglement between the polarization degrees of freedom of two photons can be interpreted as a violation of Leggett's crypto-nonlocality.
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• The role of entanglement in work extraction
Karen V. Hovhannisyan, Martí Perarnau-Llobet, Marcus Huber, Antonio Acín
20 March 2013

Abstract:
We consider reversible work extraction from identical quantum batteries. From an ensemble of individually passive states, work can be produced only via global unitary (and thus entangling) operations. However, we show here that there always exists a method to extract all possible work without creating any entanglement, at the price of generically requiring more operations (i.e. additional time). We then study faster methods to extract work and provide a quantitative relation between the amount of generated multipartite entanglement and extractable work. Our results suggest a general relation between entanglement generation and the power of work extraction.
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• Causal Fermions in Discrete Spacetime
Terence C. Farrelly, Anthony J. Short
20 March 2013

Abstract:
In this paper, we consider fermionic systems in discrete spacetime evolving with a strict notion of causality, meaning they evolve unitarily and with a bounded propagation speed. First, we show that the evolution of these systems has a natural decomposition into a product of local unitaries, which also holds if we include bosons. Next, we show that causal evolution of fermions in discrete spacetime can also be viewed as the causal evolution of a lattice of qubits, meaning these systems can be viewed as quantum cellular automata. Following this, we discuss some examples of causal fermionic models in discrete spacetime that become interesting physical systems in the continuum limit: Dirac fermions in one and three spatial dimensions, Dirac fields and briefly the Thirring model. Finally, we show that the dynamics of causal fermions in discrete spacetime can be efficiently simulated on a quantum computer.
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• Bell nonlocality
Nicolas Brunner, Daniel Cavalcanti, Stefano Pironio, Valerio Scarani, Stephanie Wehner
13 March 2013

Abstract:
Bell's 1964 theorem, which states that the predictions of quantum theory cannot be accounted for by any local theory, represents one of the most profound developments in the foundations of physics. In the last two decades, Bell's theorem has been a central theme of research from a variety of perspectives, mainly motivated by quantum information science, where the nonlocality of quantum theory underpins many of the advantages afforded by a quantum processing of information. The focus of this review is to a large extent oriented by these later developments. We review the main concepts and tools which have been developed to describe and study the nonlocality of quantum theory, and which have raised this topic to the status of a full sub-field of quantum information science.
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• The Quantum Entropy Cone of Stabiliser States
Noah Linden, Mary Beth Ruskai, Andreas Winter
25 February 2013

Abstract:
We investigate the universal linear inequalities that hold for the von Neumann entropies in a multi-party system, prepared in a stabiliser state. We demonstrate here that entropy vectors for stabiliser states satisfy, in addition to the classic inequalities, a type of linear rank inequalities associated with the combinatorial structure of normal subgroups of certain matrix groups.
In the 4-party case, there is only one such inequality, the so-called Ingleton inequality. For these systems we show that strong subadditivity, weak monotonicity and Ingleton inequality exactly characterize the entropy cone for stabiliser states.
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• Extracting work from quantum systems
Paul Skrzypczyk, Anthony J. Short, Sandu Popescu
13 February 2013

Abstract:
We consider the task of extracting work from quantum systems in the resource theory perspective of thermodynamics, where free states are arbitrary thermal states, and allowed operations are energy conserving unitary transformations. Taking as our work storage system a 'weight' we prove the second law and then present simple protocols which extract average work equal to the free energy change of the system - the same amount as in classical thermodynamics. Crucially, for systems in 'classical' states (mixtures of energy eigenstates) our protocol works on a single copy of the system. This is in sharp contrast to previous results, which showed that in case of almost-deterministic work extraction, collective actions on multiple copies are necessary to extract the free energy. This establishes the fact that free energy is a meaningful notion even for individual systems in classical states. However, for non-classical states, where coherences between energy levels exist, we prove that collective actions are necessary, so long as no external sources of coherence are used.
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• Amplification of arbitrarily weak randomness
Piotr Mironowicz, Marcin Pawlowski
01 February 2013

Abstract:
We demonstrate that amplification of arbitrarily weak randomness is possible using quantum resources. We present a randomness amplification protocol that involves Bell experiments. We report on two Bell inequalities which can amplify arbitrarily weak randomness and give detailed analysis for one of them. Our analysis includes finding a sufficient violation of Bell inequality as a function of the quality of randomness. It has a very important property that for any quality the required violation is strictly lower than possible to obtain using quantum resources. Among other things it means that the protocol takes a finite amount of time to amplify any randomness.
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• Peculiar Features of Entangled States with Post-Selection
Yakir Aharonov, Shmuel Nussinov, Sandu Popescu, Lev Vaidman
28 January 2013

Abstract:
We consider quantum systems in entangled states post-selected in non-entangled states. Such systems exhibit unusual behavior, in particular when weak measurements are performed at intermediate times.
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• Towards a strong converse for the quantum capacity (of degradable channels)
Ciara Morgan, Andreas Winter
22 January 2013

Abstract:
We exhibit a possible road towards a strong converse for the quantum capacity of degradable channels. In particular, we show that all degradable channels obey what we call a "pretty strong" converse: When the code rate increases above the quantum capacity, the fidelity makes a discontinuous jump from 1 to at most 0.707, asymptotically. A similar result can be shown for the private (classical) capacity. Furthermore, we can show that if the strong converse holds for symmetric channels (which have quantum capacity zero), then degradable channels obey the strong converse: The above-mentioned asymptotic jump of the fidelity at the quantum capacity is then from 1 down to 0.
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• Higher dimensional entanglement enables QKD without perfect randomness
Marcus Huber, Marcin Pawlowski
14 January 2013

Abstract:
A recent paper introduces an attack on quantum key distribution protocols that exploits imperfect randomness and a sublinear sample size. We show that a generalized attack compromises the security even with a linear size test sample and device independent security considerations. We explicitly derive the sample size needed to retrieve security as a function of the randomness quality. We demonstrate that exploiting features of genuinely higher dimensional systems one can reduce this weakness and provide device independent security more robust against weak randomness sources.
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• Simulating all non-signalling correlations via classical or quantum theory with negative probabilities
Sabri W. Al-Safi, Anthony J. Short
11 January 2013

Abstract:
Many-party correlations between measurement outcomes in general probabilistic theories are given by conditional probability distributions obeying the non-signalling condition. We show that any such distribution can be obtained from classical or quantum theory, by relaxing positivity constraints on either the mixed state shared by the parties, or the local functions which generate measurement outcomes. Our results apply to generic non-signalling correlations, but in particular they yield two distinct quasi-classical models for quantum correlations.
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• Quantum rate distortion coding with auxiliary resources
Nilanjana Datta, Mark M. Wilde, Min-Hsiu Hsieh, Andreas Winter
24 December 2012

Abstract:
We extend quantum rate distortion theory by considering auxiliary resources that might be available to a sender and receiver performing lossy quantum data compression. The first setting we consider is quantum rate distortion coding with the help of a classical side channel. Our result here is that the regularized entanglement of formation characterizes the quantum rate distortion function, extending earlier work of Devetak and Berger. We also combine this bound with the entanglement-assisted bound from our prior work to obtain the best known bounds on the quantum rate distortion function for an isotropic qubit source. The second setting we consider is quantum rate distortion coding with quantum side information (QSI) available to the receiver. In order to prove results in this setting, we first state and prove a quantum reverse Shannon theorem with QSI (for tensor-power states), which extends the known tensor-power quantum reverse Shannon theorem. The achievability part of this theorem relies on the quantum state redistribution protocol, while the converse relies on non-trivial entropic manipulations that are unique to quantum information theory and the fact that the protocol can cause only a negligible disturbance to the state of the reference and the receiver's QSI. This quantum reverse Shannon theorem with QSI naturally leads to quantum rate-distortion theorems with QSI, with or without entanglement assistance.
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• Testing nonlocality of single-photon entanglement without a shared reference frame
Jonatan Bohr Brask, Rafael Chaves, Nicolas Brunner
11 December 2012

Abstract:
The fact that a single photon can be in a superposition of several spatial modes leads to the concept of single-photon entanglement, which has been much debated in the past. Here we discuss a simple scheme for revealing the nonlocality (hence also the entanglement) of a single-photon, which is analogous to the usual case of multiparticle entanglement. The most attractive feature of our scheme is that it does not require the separated observers to share a common reference frame. Specifically, Bell inequality violation can be obtained with certainty with unaligned devices, even if the relative frame fluctuates between each experimental run of the Bell test. These ideas are relevant from an experimental viewpoint and may significantly simplify the realization of quantum communication protocols based on single-photon entanglement.
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• Everything You Always Wanted to Know About LOCC (But Were Afraid to Ask)
Eric Chitambar, Debbie Leung, Laura Mancinska, Maris Ozols, Andreas Winter
18 October 2012

Abstract:
In this paper we study the subset of generalized quantum measurements on finite dimensional systems known as local operations and classical communication (LOCC). While LOCC emerges as the natural class of operations in many important quantum information tasks, its mathematical structure is complex and difficult to characterize. Here we provide a precise description of LOCC and related operational classes in terms of quantum instruments. Our formalism captures both finite round protocols as well as those that utilize an unbounded number of communication rounds. While the set of LOCC is not topologically closed, we show that finite round LOCC constitutes a compact subset of quantum operations. Finally, we demonstrate a two-qubit map whose action can be approached arbitrarily close using LOCC, but nevertheless cannot be implemented perfectly.
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• Self Testing of Partially Entangled States
Tzyh Haur Yang, Miguel Navascues
17 October 2012

Abstract:
Self testing is a device independent approach to estimate the state and measurement operators, without the need to assume the dimension of our quantum system. In this paper, we show that one can self test any pure entangled two-qubit state, by performing simple Bell type experiments. The approach makes use of only one family of two-inputs/two-outputs Bell inequalities. Furthermore, we outline the sufficient conditions for one to self test any dimensional bipartite entangled state. All these methods are robust to small but inevitable experimental errors.
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• Quantum steering with bound entanglement
Nicolas Brunner, Daniel Cavalcanti
05 October 2012

Abstract:
Quantum steering, loosely speaking the distribution of entanglement from an untrusted party, is a form of quantum nonlocality which is intermediate between entanglement and Bell nonlocality. Determining which states can be steered is important from a conceptual point of view, but also for applications, e.g. in quantum cryptography. Here we show that bound entanglement, although it represents the weakest form of entanglement, can nevertheless lead to quantum steering. This is done by noticing that steering inequalities can be derived from entropic uncertainty relations. Our result has implications on the connection between entanglement distillability and nonlocality, and shows that bound entangled states can be useful for information-theoretic tasks featuring an untrusted party.
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• Attacks on semi-device independent quantum protocols
Michele Dall'Arno, Elsa Passaro, Rodrigo Gallego, Marcin Pawlowski, Antonio Acin
05 October 2012

Abstract:
Recently several semi-device independent quantum protocols were proposed - e.g. for secure key distribution, random access coding, and randomness generation - in a scenario where no assumption on the internal working of the devices used in the protocol is made, except their dimension. These protocols, while being often more practical than fully-device independent ones, are also clearly more secure than their device dependent counterparts. Nevertheless, we discuss conditions under which detection inefficiencies can be exploited to fake the result of the protocol - and how to prevent it - in terms of the detection probability and of the worst case success probability of a random access code.
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• Bell nonlocality and Bayesian game theory
Nicolas Brunner, Noah Linden
04 October 2012

Abstract:
We discuss a connection between Bell nonlocality and Bayesian games. This link offers interesting perspectives for Bayesian games, namely to allow the players to receive advice in the form of nonlocal correlations, for instance using entangled quantum particles or more general no-signaling boxes. The possibility of having such 'nonlocal advice' will lead to novel joint strategies, impossible to achieve in the classical setting. This implies that quantum resources, or more general no-signaling resources, offer a genuine advantage over classical ones. Moreover, some of these strategies can represent equilibrium points, leading to the notion of quantum/no-signaling Nash equilibrium. Finally we describe new types of question in the study of nonlocality, namely the consideration of non-local advantage when there is a set of Bell expressions.
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• Systems of Imprimitivity for the Clifford Group
D. M. Appleby, Ingemar Bengtsson, Stephen Brierley, Åsa Ericsson, Markus Grassl, Jan-Åke Larsson
04 October 2012

Abstract:
It is known that if the dimension is a perfect square the Clifford group can be represented by monomial matrices. Another way of expressing this result is to say that when the dimension is a perfect square the standard representation of the Clifford group has a system of imprimitivity consisting of one dimensional subspaces. We generalize this result to the case of an arbitrary dimension. Let k be the square-free part of the dimension. Then we show that the standard representation of the Clifford group has a system of imprimitivity consisting of k-dimensional subspaces. To illustrate the use of this result we apply it to the calculation of SIC-POVMs (symmetric informationally complete positive operator valued measures), constructing exact solutions in dimensions 8 (hand-calculation) as well as 12 and 28 (machine-calculation).
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• On the relation between semi and fully device independent protocols
Hong-Wei Li, Piotr Mironowicz, Marcin Pawlowski, Zhen-Qiang Yin, Yu-Chun Wu, Shuang Wang, Wei Chen, Hong-Gang Hu, Guang-Can Guo, Zheng-Fu Han
02 October 2012

Abstract:
We study the relation between semi and fully device independent protocols. Our work leads to new random number generating protocols with higher rates for both scenarios. As a tool we use the correspondence between Bell inequalities and dimension witnesses. We present a method for converting the former into the later and vice versa. As a byproduct we obtain whole new classes of inequalities and witnesses. Finally, we show how optimization methods used in the studies of Bell inequalities can be adopted for dimension witnesses.
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• Dimension witnesses and quantum state discrimination
Nicolas Brunner, Miguel Navascués, Tamás Vértesi
26 September 2012

Abstract:
Dimension witnesses allow one to test the dimension of an unknown physical system in a device-independent manner, that is, without placing assumptions about the functioning of the devices used in the experiment. Here we present simple and general dimension witnesses for quantum systems of arbitrary Hilbert space dimension. Our approach is deeply connected to the problem of quantum state discrimination, hence establishing a strong link between these two research topics. Finally, our dimension witnesses can distinguish between classical and quantum systems of the same dimension, making them potentially useful for quantum information processing.
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• Coherent Time Evolution and Boundary Conditions of Two-Photon Quantum Walks
Jasmin D. A. Meinecke, Kostas Poulios, Alberto Politi, Jonathan C. F. Matthews, Alberto Peruzzo, Nur Ismail, Kerstin Wörhoff, Jeremy L. O'Brien, Mark G. Thompson
10 September 2012

Abstract:
Multi-photon quantum walks in integrated optics are an attractive controlled quantum system, that can mimic less readily accessible quantum systems and exhibit behavior that cannot in general be accurately replicated by classical light without an exponential overhead in resources. The ability to observe time evolution of such systems is important for characterising multi-particle quantum dynamics---notably this includes the effects of boundary conditions for walks in spaces of finite size. Here we demonstrate the coherent evolution of quantum walks of two indistinguishable photons using planar arrays of 21 evanescently coupled waveguides fabricated in silicon oxynitride technology. We compare three time evolutions, that follow closely a model assuming unitary evolution, corresponding to three different lengths of the array---in each case we observe quantum interference features that violate classical predictions. The longest array includes reflecting boundary conditions.
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• Super-stable tomography of any linear optical device
Anthony Laing, Jeremy L. O'Brien
15 August 2012

Abstract:
Linear optical circuits of growing complexity are playing an increasing role in emerging photonic quantum technologies. Individual photonic devices are typically described by a unitary matrix containing amplitude and phase information, the characterisation of which is a key task. We present a constructive scheme to retrieve the unitary matrix describing an arbitrary linear optical device using data obtained from one-photon and two-photon ensembles. The scheme is stable on the arbitrarily increasable length scale of the photon packet and independent of photon loss at input and output ports of the device. We find a one-to-one correspondence between ideal data and unitary matrix, and identify the class of non-unitary matrices capable of reproducing the data. The method is extended for coherent state probes, which can simulate two-photon statistics with a reduced visibility. We analyse the performance of reconstruction to simulated noise.
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• Genuinely Multipartite Concurrence of N-qubit X-matrices
S. M. Hashemi Rafsanjani, M. Huber, C. J. Broadbent, J. H. Eberly
15 August 2012

Abstract:
We find an algebraic formula for the N-partite concurrence of N qubits in an X-matrix. X- matricies are density matrices whose only non-zero elements are diagonal or anti-diagonal when written in an orthonormal basis. We use our formula to study the dynamics of the N-partite entanglement of N remote qubits in generalized N-party Greenberger-Horne-Zeilinger (GHZ) states. We study the case when each qubit interacts with a partner harmonic oscillator. It is shown that only one type of GHZ state is prone to entanglement sudden death; for the rest, N-partite entanglement dies out momentarily. Algebraic formulas for the entanglement dynamics are given in both cases.
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• Closing the detection loophole in multipartite Bell tests using GHZ states
Karoly F. Pal, Tamas Vertesi, Nicolas Brunner
06 August 2012

Abstract:
We investigate the problem of closing the detection loophole in multipartite Bell tests, and show that the required detection efficiencies can be significantly lowered compared to the bipartite case. In particular, we present Bell tests based on n-qubit Greenberger-Horne-Zeilinger states, which can tolerate efficiencies as low as 38% for a reasonable number of parties and measurements. Even in the presence of a significant amount of noise, efficiencies below 50% can be tolerated, which is encouraging given recent experimental progress. Finally we give strong evidence that, for a sufficiently large number of parties and measurements, arbitrarily small efficiencies can be tolerated, even in the presence of an arbitrary large amount of noise.
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• Generating entanglement with low Q-factor microcavities
A. B. Young, C. Y. Hu, J. G. Rarity
03 August 2012

Abstract:
We propose a method of generating entanglement using single photons and electron spins in the regime of resonance scattering. The technique involves matching the spontaneous emission rate of the spin dipole transition in bulk dielectric to the modified rate of spontaneous emission of the dipole coupled to the fundamental mode of an optical microcavity. We call this regime resonance scattering where interference between the input photons and those scattered by the resonantly coupled dipole transition result in a reflectivity of zero. The contrast between this and the unit reflectivity when the cavity is empty allow us to perform a non demolition measurement of the spin and to non deterministically generate entanglement between photons and spins. The chief advantage of working in the regime of resonance scattering is that the required cavity quality factors are orders of magnitude lower than is required for strong coupling, or Purcell enhancement. This makes engineering a suitable cavity much easier particularly in materials such as diamond where etching high quality factor cavities remains a significant challenge.
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• Super-activation of quantum nonlocality and teleportation
Daniel Cavalcanti, Antonio Acin, Nicolas Brunner, Tamas Vertesi
24 July 2012

Abstract:
Recently, Palazuelos presented the first example of super-activation of quantum nonlocality. More precisely, by combining several copies of a state rho admitting a local hidden variable model, it becomes possible to violate a Bell inequality, in a deterministic way. Here we show that super-activation of quantum nonlocality is in fact a rather generic phenomenon. In particular, we show that any d x d entangled state with entanglement fraction larger than 1/d can be super-activated. Remarkably this implies that all entangled states which are useful for teleportation are nonlocal resources, hence providing a direct link between teleportation and nonlocality.
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• Persistency of multipartite quantum correlations
Nicolas Brunner, Tamas Vertesi
18 July 2012

Abstract:
The behaviour under particle loss of entanglement and nonlocality is investigated in multipartite quantum systems. In particular, we define a notion of persistency of nonlocality, which leads to device-independent tests of persistent entanglement. We investigate the persistency of various classes of multipartite quantum states, exhibiting a variety of different behaviours. A particular attention is devoted to states featuring maximal persistency. Finally we discuss a link between the symmetry of a state and its persistency, illustrating the fact that too much symmetry reduces the strength of correlations among subsystems. These ideas also lead to a device-independent estimation of the asymmetry of a quantum state.
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• Measurement entropy in Generalized Non-Signalling Theory cannot detect bipartite non-locality
Josh Cadney, Noah Linden
16 July 2012

Abstract:
We consider entropy in Generalized Non-Signalling Theory (also known as box world) where the most common definition of entropy is the measurement entropy. In this setting, we completely characterize the set of allowed entropies for a bipartite state. We find that the only inequalities amongst these entropies are subadditivity and non-negativity. What is surprising is that non-locality does not play a role - in fact any bipartite entropy vector can be achieved by separable states of the theory. This is in stark contrast to the case of the von Neumann entropy in quantum theory, where only entangled states satisfy S(AB)<S(A).
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• Efficient Distributed Quantum Computing
Robert Beals, Stephen Brierley, Oliver Gray, Aram Harrow, Samuel Kutin, Noah Linden, Dan Shepherd, Mark Stather
10 July 2012

Abstract:
We provide algorithms for efficiently addressing quantum memory in parallel. These imply that the standard circuit model can be simulated with low overhead by the more realistic model of a distributed quantum computer. As a result, the circuit model can be used by algorithm designers without worrying whether the underlying architecture supports the connectivity of the circuit. In addition, we apply our results to existing memory intensive quantum algorithms. We present a parallel quantum search algorithm and improve the time-space trade-off for the Element Distinctness and Collision problems.
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• Quantum gates and multipartite entanglement resonances realized by motion
Nicolai Friis, Marcus Huber, Ivette Fuentes, David Edward Bruschi
07 July 2012

Abstract:
We demonstrate the presence of genuine multipartite entanglement between the modes of quantum fields in non-uniformly moving cavities. The transformations generated by the cavity motion can be considered as multipartite quantum gates. We present two setups for which multi-mode entanglement can be generated for bosons and fermions. As a highlight we show that the bosonic genuine multipartite correlations can be resonantly enhanced. Our results provide fundamental insights into the structure of Bogoliubov transformations and suggest strong links between quantum information, quantum fields in curved spacetimes and gravitational analogs by way of the equivalence principle.
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• Interfacing a quantum dot spin with a photonic circuit
Isaac J. Luxmoore, Nicholas A. Wasley, Andrew J. Ramsay, Arthur C. T. Thijssen, Ruth Oulton, Maxime Hugues, Sachin Kasture, Achanta V. Gopal, A. Mark Fox, Maurice S. Skolnick
14 June 2012

Abstract:
A scalable optical quantum information processor is likely to be a waveguide circuit with integrated sources, detectors, and either deterministic quantum-logic or quantum memory elements. With microsecond coherence times, ultrafast coherent control, and lifetime-limited transitions, semiconductor quantum-dot spins are a natural choice for the static qubits. However their integration with flying photonic qubits requires an on-chip spin-photon interface, which presents a fundamental problem: the spin-state is measured and controlled via circularly-polarised photons, but waveguides support only linear polarisation. We demonstrate here a solution based on two orthogonal photonic nanowires, in which the spin-state is mapped to a path-encoded photon, thus providing a blue-print for a scalable spin-photon network. Furthermore, for some devices we observe that the circular polarisation state is directly mapped to orthogonal nanowires. This result, which is physically surprising for a non-chiral structure, is shown to be related to the nano-positioning of the quantum-dot with respect to the photonic circuit.
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• Distinguishing multi-partite states by local measurements
Cécilia Lancien, Andreas Winter
13 June 2012

Abstract:
We analyze the distinguishability norm on the states of a multi-partite system, defined by local measurements. Concretely, we show that the norm associated to a tensor product of sufficiently symmetric measurements is essentially equivalent to a multi-partite generalisation of the non-commutative 2-norm (aka Hilbert-Schmidt norm): in comparing the two, the constants of domination depend only on the number of parties but not on the Hilbert spaces dimensions. We discuss implications of this result on the corresponding norms for the class of all measurements implementable by local operations and classical communication (LOCC), and in particular on the leading order optimality of multi-party data hiding schemes.
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• Non-Additivity of the Entanglement of Purification (Beyond Reasonable Doubt)
Jianxin Chen, Andreas Winter
06 June 2012

Abstract:
We demonstrate the convexity of the difference between the regularized entanglement of purification and the entropy, as a function of the state. This is proved by means of a new asymptotic protocol to prepare a state from pre-shared entanglement and by local operations only. We go on to employ this convexity property in an investigation of the additivity of the (single-copy) entanglement of purification: using numerical results for two-qubit Werner states we find strong evidence that the entanglement of purification is different from its regularization, hence that entanglement of purification is not additive.
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• Reply to "Comment on 'Security proof for cryptographic protocols based only on the monogamy of Bell's inequality violations' "
M. Pawlowski
30 May 2012

Abstract:
In this reply I address the comment by W-Y. Hwang and O. Gittsovich on my paper [Phys. Rev. A {\bf 82}, 032313 (2010)]. The authors of the comment point out that I use implicit assumption that the alphabet of the eavesdropper is binary. They claim that such assumption is unrealistic. Here I show that even without this assumption the main result of my paper still holds.
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• A quantum delayed choice experiment
Alberto Peruzzo, Peter J. Shadbolt, Nicolas Brunner, Sandu Popescu, Jeremy L. O'Brien
22 May 2012

Abstract:
Quantum systems exhibit particle-like or wave-like behaviour depending on the experimental apparatus they are confronted by. This wave-particle duality is at the heart of quantum mechanics, and is fully captured in Wheeler's famous delayed choice gedanken experiment. In this variant of the double slit experiment, the observer chooses to test either the particle or wave nature of a photon after it has passed through the slits. Here we report on a quantum delayed choice experiment, based on a quantum controlled beam-splitter, in which both particle and wave behaviours can be investigated simultaneously. The genuinely quantum nature of the photon's behaviour is tested via a Bell inequality, which here replaces the delayed choice of the observer. We observe strong Bell inequality violations, thus showing that no model in which the photon knows in advance what type of experiment it will be confronted by, hence behaving either as a particle or as wave, can account for the experimental data.
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• Observation of quantum interference as a function of Berry's phase in a complex Hadamard optical network
Anthony Laing, Thomas Lawson, Enrique Martín López, Jeremy L. O'Brien
21 May 2012

Abstract:
Emerging models of quantum computation driven by multi-photon quantum interference, while not universal, may offer an exponential advantage over classical computers for certain problems. Implementing these circuits via geometric phase gates could mitigate requirements for error correction to achieve fault tolerance while retaining their relative physical simplicity. We report an experiment in which a geometric phase is embedded in an optical network with no closed-loops, enabling quantum interference between two photons as a function of the phase.
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• Reconfigurable controlled two-qubit operation on a quantum photonic chip
H. W. Li, S. Przeslak, A. O. Niskanen, J. C. F. Matthews, A. Politi, P. Shadbolt, A. Laing, M. Lobino, M. G. Thompson, J. L. O'Brien
19 May 2012

Abstract:
Integrated quantum photonics is an appealing platform for quantum information processing, quantum communication and quantum metrology. In all these applications it is necessary not only to be able to create and detect Fock states of light but also to program the photonic circuits that implements some desired logical operation. Here we demonstrate a reconfigurable controlled two-qubit operation on a chip using a multiwaveguide interferometer with a tunable phase shifter. We find excellent agreement between theory and experiment, with a 0.98 \pm 0.02 average similarity between measured and ideal operations.
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• Joint system quantum descriptions arising from local quantumness
Tom Cooney, Marius Junge, Miguel Navascues, David Perez-Garcia, Ignacio Villanueva
18 May 2012

Abstract:
Bipartite correlations generated by non-signalling physical systems that admit a finite-dimensional local quantum description cannot exceed the quantum limits, i.e., they can always be interpreted as distant measurements of a bipartite quantum state. Here we consider the effect of dropping the assumption of finite dimensionality. Remarkably, we find that the same result holds provided that we relax the tensor structure of space-like separated measurements to mere commutativity. We argue why an extension of this result to tensor representations seems unlikely.
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• Determining lower bounds on a measure of multipartite entanglement from few local observables
Jun-Yi Wu, Hermann Kampermann, Dagmar Bruß, Claude Klöckl, Marcus Huber
14 May 2012

Abstract:
We introduce a method to lower bound an entropy based measure of genuine multipartite entanglement via nonlinear entanglement witnesses. We show that some of these bounds are tight and explicitly work out their connection to a framework of nonlinear witnesses that were published recently. Furthermore we provide a detailed analysis of these lower bounds in the context of other possible bounds and measures. In exemplary cases we show that only few local measurements are necessary to determine these lower bounds.
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• Guess your neighbour's input: no quantum advantage but an advantage for quantum theory
Antonio Acín, Mafalda L. Almeida, Remigiusz Augusiak, Nicolas Brunner
14 May 2012

Abstract:
Quantum mechanics dramatically differs from classical physics, allowing for a wide range of genuinely quantum phenomena. The goal of quantum information is to understand information processing from a quantum perspective. In this mindset, it is thus natural to focus on tasks where quantum resources provide an advantage over classical ones, and to overlook tasks where quantum mechanics provides no advantage. But are the latter tasks really useless from a more general perspective? Here we discuss a simple information-theoretic game called 'guess your neighbour's input', for which classical and quantum players perform equally well. We will see that this seemingly innocuous game turns out to be useful in various contexts. From a fundamental point of view, the game provides a sharp separation between quantum mechanics and other more general physical theories, hence bringing a deeper understanding of the foundations of quantum mechanics. The game also finds unexpected applications in quantum foundations and quantum information theory, related to Gleason's theorem, and to bound entanglement and unextendible product bases.
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• Entanglement of Photons with Tunable Singularities
Mario Krenn, Robert Fickler, Marcus Huber, Radek Lapkiewicz, William Plick, Sven Ramelow, Anton Zeilinger
11 May 2012

Abstract:
Photons with higher order spatial structures open up the possibility for fundamental highdimensional quantum experiments and quantum information tasks. The most commonly used set of this type is the Laguerre-Gauss family - rotationally symmetric modes with a central singularity that carry orbital angular momentum. Here we show for the first time entanglement of the generalised elliptically-symmetric modes called Ince-Gauss. The Laguerre-Gauss modes emerge as a special case for vanishing ellipticity. For nonzero ellipticity the central singularity splits into individual singularities. Their positions as well as those of additional singularities in the outer parts of the modes can be tuned by modifying the ellipticity. We verify 2-dimensional and 3-dimensional entanglement of Ince-Gauss modes and measure an Ince-Gauss specific quantum-correlation function with possible use in future quantum communication protocols. These entanglement experiments confirm that when one photon is projected into a state with a specific singularity pattern, its entangled partner is also projected into a specific singularity pattern while neither carried such singularities before.
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• Quantum Steering and Space-Like Separation
Miguel Navascues, David Perez-Garcia
27 April 2012

Abstract:
In non-relativistic quantum mechanics, measurements performed by separate observers are modeled via tensor products. In Algebraic Quantum Field Theory, though, local observables corresponding to space-like separated parties are just required to commute. The problem of determining whether these two definitions of "separation" lead to the same set of bipartite correlations is known in non-locality as Tsirelson's problem. In this article, we prove that the analog of Tsirelson's problem in steering scenarios is false. That is, there exists a steering inequality that can be violated or not depending on how we define space-like separation at the operator level.
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• A high-brightness source of polarization-entangled photons optimized for applications in free space
Fabian Steinlechner, Pavel Trojek, Marc Jofre, Henning Weier, Daniel Perez, Thomas Jennewein, Rupert Ursin, John Rarity, Morgan W. Mitchell, Juan P. Torres, Harald Weinfurter, Valerio Pruneri
24 April 2012

Abstract:
We present a simple but highly efficient source of polarization-entangled photons based on spontaneous parametric down-conversion (SPDC) in bulk periodically poled potassium titanyl phosphate crystals (PPKTP) pumped by a 405 nm laser diode. Utilizing one of the highest available nonlinear coefficients in a non-degenerate, collinear type-0 phase-matching configuration, we generate polarization entanglement via the crossed-crystal scheme and detect 0.64 million photon pair events/s/mW, while maintaining an overlap fidelity with the ideal Bell state of 0.98 at a pump power of 0.025 mW.
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• Photon Pair Generation in Silicon Micro-Ring Resonator and Enhancement via Reverse Bias
Erman Engin, Damien Bonneau, Chandra M. Natarajan, Alex Clark, M. G. Tanner, R. H. Hadfield, Sanders N. Dorenbos, Val Zwiller, Kazuya Ohira, Nobuo Suzuki, Haruhiko Yoshida, Norio Iizuka, Mizunori Ezaki, Jeremy L. O'Brien, Mark G. Thompson
22 April 2012

Abstract:
We experimentally demonstrate photon pair generation through spontaneous four-wave mixing in a silicon micro-ring resonator. We report a coincidence-to-accidental (CAR) ratio of 456\pm18 using the full width half maximum of the coincidence histogram as the integration window. In order to overcome the free-carrier related performance degradation we have investigated reverse biasing the ring. We show that this method improves the pair generation rate by a factor of up to 2.1.
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• Open Quantum System Approach to the Modeling of Spin Recombination Reactions
M. Tiersch, U. E. Steiner, S. Popescu, H. J. Briegel
20 April 2012

Abstract:
In theories of spin-dependent radical pair reactions, the time evolution of the radical pair, including the effect of the chemical kinetics, is described by a master equation in the Liouville formalism. For the description of the chemical kinetics, a number of possible reaction operators have been formulated in the literature. In this work, we present a framework that allows for a unified description of the various proposed mechanisms and the forms of reaction operators for the spin-selective recombination processes. Based on the concept that master equations can be derived from a microscopic description of the spin system interacting with external degrees of freedom, it is possible to gain insight into the underlying microscopic processes and to develop a systematic approach towards determining the specific form of reaction operator in concrete scenarios.
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• Bell inequalities for three systems and arbitrarily many measurement outcomes
Basile Grandjean, Yeong-Cherng Liang, Jean-Daniel Bancal, Nicolas Brunner, Nicolas Gisin
17 April 2012

Abstract:
We present a family of Bell inequalities for three parties and arbitrarily many outcomes, which can be seen as a natural generalization of the Mermin Bell inequality. For a small number of outcomes, we verify that our inequalities define facets of the polytope of local correlations. We investigate the quantum violations of these inequalities, in particular with respect to the Hilbert space dimension. We provide strong evidence that the maximal quantum violation can only be reached using systems with local Hilbert space dimension exceeding the number of measurement outcomes. This suggests that our inequalities can be used as multipartite dimension witnesses.
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• Quantum state discrimination bounds for finite sample size
Koenraad M.R. Audenaert, Milan Mosonyi, Frank Verstraete
03 April 2012

Abstract:
In the problem of quantum state discrimination, one has to determine by measurements the state of a quantum system, based on the a priori side information that the true state is one of two given and completely known states, rho or sigma. In general, it is not possible to decide the identity of the true state with certainty, and the optimal measurement strategy depends on whether the two possible errors (mistaking rho for sigma, or the other way around) are treated as of equal importance or not. Recent results on the quantum Chernoff and Hoeffding bounds show that, if several copies of the system are available then the optimal error probabilities decay exponentially in the number of copies, and the decay rate is given by a certain statistical distance between rho and sigma (the Chernoff distance and the Hoeffding distances, respectively). While these results provide a complete solution for the asymptotic problem, they are not completely satisfying from a practical point of view. Indeed, in realistic scenarios one has access only to finitely many copies of a system, and therefore it is desirable to have bounds on the error probabilities for finite sample size. In this paper we provide finite-size bounds on the so-called Stein errors, the Chernoff errors, the Hoeffding errors and the mixed error probabilities related to the Chernoff and the Hoeffding errors.
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• The Complete Quantum Cheshire Cat
Yelena Guryanova, Nicolas Brunner, Sandu Popescu
19 March 2012

Abstract:
We show that a physical property can be entirely separated from the object it belongs to, hence realizing a complete quantum Cheshire cat. Our setup makes use of a type of quantum state of particular interest, namely an entangled pre- and post-selected state, in which the pre- and post-selections are entangled with each other. Finally we propose a scheme for the experimental implementation of these ideas.
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• A paradox in bosonic energy calculations via semidefinite programming relaxations
M. Navascues, A. Garcia-Saez, A. Acin, S. Pironio, M. B. Plenio
16 March 2012

Abstract:
We show that the recent hierarchy of semidefinite programming approximations based on non-commutative polynomial optimization and reduced density matrix variational methods exhibits an interesting paradox when extended to the bosonic case: even though it can be proven that the hierarchy collapses after the first step, one finds numerically that higher order steps generate a sequence of lower bounds that converges to the optimal solution. We analyze this effect and compare it with similar behavior observed in implementations of semidefinite programming relaxations for classical polynomial minimization. We conclude that the method converges due to the rounding errors occurring during the execution of the numerical program, and show that convergence is lost as soon as computer precision is incremented. We support this conclusion by proving that for any element p of a Weyl algebra which is non-negative in the Schrodinger representation there exists another element p arbitrarily close to p that admits a sum of squares decomposition.
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• Robust Self Testing of the Singlet
Matthew McKague, Tzyh Haur Yang, Valerio Scarani
13 March 2012

Abstract:
In this paper, we introduce a general framework to study the concept of robust self testing which can be used to self test EPR pairs and local measurement operators. The result is based only on probabilities obtained from experiment, with tolerance to experimental errors. In particular, we show that if results of experiment come approach the Cirel'son bound, or approximates the Mayers-Yao type correlation, then the experiment must contain an approximate EPR pair. More specifically, there exist local bases in which the physical state is close to an EPR pair, possibly all encoded in a larger environment or ancilla. Moreover, in theses bases the measurements are close to the qubit operators used to achieve the Cirel'son bound or the Mayers-Yao results.
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• Classical simulation of entanglement swapping with bounded communication
Cyril Branciard, Nicolas Brunner, Harry Buhrman, Richard Cleve, Nicolas Gisin, Samuel Portmann, Denis Rosset, Mario Szegedy
02 March 2012

Abstract:
Entanglement appears under two different forms in quantum theory, namely as a property of states of joint systems and as a property of measurement eigenstates in joint measurements. By combining these two aspects of entanglement, it is possible to generate nonlocality between particles that never interacted, using the protocol of entanglement swapping. We show that even in the more constraining bilocal scenario where distant sources of particles are assumed to be independent, i.e. to share no prior randomness, this process can be simulated classically with bounded communication, using only 9 bits in total. Our result thus provides an upper bound on the nonlocality of the process of entanglement swapping.
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• Bell tests for continuous variable systems using hybrid measurements and heralded amplifiers
Jonatan Bohr Brask, Nicolas Brunner, Daniel Cavalcanti, Anthony Leverrier
01 March 2012

Abstract:
We present Bell tests for optical continuous variable systems, combining both hybrid measurements (i.e. measuring both particle and wave aspects of light) and heralded amplifiers. We discuss two types of schemes, in which the amplifier is located either at the source, or at the parties' laboratories. The inclusion of amplifiers helps to reduce the detrimental effect of losses in the setup. In particular, we show that the requirements in terms of detection efficiency and transmission losses are significantly reduced, approaching the experimentally accessible regime.
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• Entanglement detection via mutually unbiased bases
Christoph Spengler, Marcus Huber, Stephen Brierley, Theodor Adaktylos, Beatrix C. Hiesmayr
22 February 2012

Abstract:
We investigate correlations among complementary observables. In particular, we show how to take advantage of mutually unbiased bases (MUBs) for the detection of entanglement in arbitrarily high-dimensional quantum systems. It is shown that their properties can be exploited to construct entanglement criteria which are experimentally implementable with few local measurement settings. The introduced concepts are not restricted to bipartite finite-dimensional systems, but are also applicable to continuous variables and multipartite systems. This is demonstrated by two examples -- the two-mode squeezed state and the Aharonov state. In addition, and more importantly from a theoretical point of view, we find a link between the separability problem and the maximum number of mutually unbiased bases.
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• GaN directional couplers for integrated quantum photonics
Yanfeng Zhang, Loyd McKnight, Erman Engin, Ian M. Watson, Martin J. Cryan, Erdan Gu, Mark G. Thompson, Stephane Calvez, Jeremy L. O'Brien, Martin D. Dawson
20 February 2012

Abstract:
Large cross-section GaN waveguides are proposed as a suitable architecture to achieve integrated quantum photonic circuits. Directional couplers with this geometry have been designed with aid of the beam propagation method and fabricated using inductively coupled plasma etching. Scanning electron microscopy inspection shows high quality facets for end coupling and a well defined gap between rib pairs in the coupling region. Optical characterization at 800 nm shows single-mode operation and coupling-length-dependent splitting ratios. Two photon interference of degenerate photon pairs has been observed in the directional coupler by measurement of the Hong-Ou-Mandel dip with 96% visibility.
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• Quantum Cheshire Cats
Yakir Aharonov, Sandu Popescu, Paul Skrzypczyk
03 February 2012

Abstract:
In this paper we present a quantum Cheshire cat. In a pre- and post-selected experiment we find the cat in one place, and the smile in another. The cat is a photon, while the smile is it's circular polarisation.
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• Quantum interference and manipulation of entanglement in silicon wire waveguide quantum circuits
Damien Bonneau, Erman Engin, Kazuya Ohira, Nob Suzuki, Haruhiko Yoshida, Norio Iizuka, Mizunori Ezaki, Chandra M. Natarajan, Michael G. Tanner, Robert H. Hadfield, Sanders N. Dorenbos, Val Zwiller, Jeremy L. O'Brien, Mark G. Thompson
31 January 2012

Abstract:
Integrated quantum photonic waveguide circuits are a promising approach to realizing future photonic quantum technologies. Here, we present an integrated photonic quantum technology platform utilising the silicon-on-insulator material system, where quantum interference and the manipulation of quantum states of light are demonstrated in components orders of magnitude smaller than in previous implementations. Two-photon quantum interference is presented in a multi-mode interference coupler, and manipulation of entanglement is demonstrated in a Mach-Zehnder interferometer, opening the way to an all-silicon photonic quantum technology platform.
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• A framework for the study of symmetric full-correlation Bell-like inequalities
Jean-Daniel Bancal, Cyril Branciard, Nicolas Brunner, Nicolas Gisin, Yeong-Cherng Liang
10 January 2012

Abstract:
Full-correlation Bell-like inequalities represent an important subclass of Bell-like inequalities that have found applications in both a better understanding of fundamental physics and in quantum information science. Loosely speaking, these are inequalities where only measurement statistics involving all parties play a role. In this paper, we provide a framework for the study of a large family of such inequalities that are symmetrical with respect to arbitrary permutations of the parties. As an illustration of the power of our framework, we derive (i) a new family of Svetlichny inequalities for arbitrary numbers of parties, settings and outcomes, (ii) a new family of two-outcome device-independent entanglement witnesses for genuine n-partite entanglement and (iii) a new family of two-outcome Tsirelson inequalities for arbitrary numbers of parties and settings. We also discuss briefly the application of these new inequalities in the characterization of quantum correlations.
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• Implementation of an iterative quantum order finding algorithm
Enrique Martin Lopez, Anthony Laing, Thomas Lawson, Roberto Alvarez, Xiao-Qi Zhou, Jeremy L. O'Brien
18 November 2011

Abstract:
Quantum algorithms are computational routines that exploit quantum mechanics to solve problems exponentially faster than the best classical algorithms. The quantum order finding algorithm is a key example---it is the subroutine that delivers the exponential speed-up in Shor's factoring algorithm. To date, the demand on quantum resources---qubits and logic gates---even for small instances, has meant that there have been only four experimental realisations. We demonstrate a scalable, iterative order finding algorithm that uses one third the number of qubits in a scalable way. Encoding in higher-dimensional states, we implement a two-photon compiled algorithm for which the algorithmic output exhibits structure that is sensitive to noise, in contrast to previous demonstrations. These results point to larger-scale implementations of Shor's algorithm by harnessing substantial but scalable resource reductions applicable to all physical architectures.
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• Persistent dynamic entanglement from classical motion: How bio-molecular machines can generate non-trivial quantum states
Gian Giacomo Guerreschi, Jianming Cai, Sandu Popescu, Hans J. Briegel
10 November 2011

Abstract:
Very recently [Phys. Rev. E 82, 021921 (2010)] a simple mechanism was presented by which a molecule subjected to forced oscillations, out of thermal equilibrium, can maintain quantum entanglement between two of its quantum degrees of freedom. Crucially, entanglement can be maintained even in the presence of very intense noise, so intense that no entanglement is possible when the forced oscillations cease. This mechanism may allow for the presence of non-trivial quantum entanglement in biological systems. Here we significantly enlarge the study of this model. In particular, we show that the persistent generation of dynamic entanglement is not restricted to the bosonic heat bath model, but it can also be observed in other decoherence models, e.g. the spin gas model, and in non-Markovian scenarios. We also show how conformational changes can be used by an elementary machine to generate entanglement even in unfavorable conditions. In biological systems, similar mechanisms could be exploited by more complex molecular machines or motors.
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• Guaranteed violation of a Bell inequality without aligned reference frames or calibrated devices
Peter Shadbolt, Tamas Vertesi, Yeong-Cherng Liang, Cyril Branciard, Nicolas Brunner, Jeremy L. O'Brien
09 November 2011

Abstract:
Bell tests---the experimental demonstration of a Bell inequality violation---are central to understanding the foundations of quantum mechanics, underpin quantum technologies, and are a powerful diagnostic tool for technological developments in these areas. To date, Bell tests have relied on careful calibration of the measurement devices and alignment of a shared reference frame between the two parties---both technically demanding tasks in general. Surprisingly, we show that neither of these operations are necessary, violating Bell inequalities with near certainty with (i) unaligned, but calibrated, measurement devices, and (ii) uncalibrated and unaligned devices. We demonstrate generic quantum nonlocality with randomly chosen local measurements on a singlet state of two photons implemented with reconfigurable integrated optical waveguide circuits based on voltage-controlled phase shifters. The observed results demonstrate the robustness of our schemes to imperfections and statistical noise. This new approach is likely to have important applications in both fundamental science and in quantum technologies, including device independent quantum key distribution.
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• Experimental estimation of the dimension of classical and quantum systems
Martin Hendrych, Rodrigo Gallego, Michal Mičuda, Nicolas Brunner, Antonio Acín, Juan P. Torres
07 November 2011

Abstract:
An overwhelming majority of experiments in classical and quantum physics make a priori assumptions about the dimension of the system under consideration. However, would it be possible to assess the dimension of a completely unknown system only from the results of measurements performed on it, without any extra assumption? The concept of a dimension witness \cite{brunner, perezgarcia, wehner, perez, gbha} answers this question, as it allows one to bound the dimension of an unknown classical or quantum system in a device-independent manner, that is, only from the statistics of measurements performed on it. Here, we report on the experimental demonstration of dimension witnesses in a prepare and measure scenario \cite{gbha}. We use pairs of photons entangled in both polarization and orbital angular momentum \cite{molina-terriza2007,mair2001} to generate ensembles of classical and quantum states of dimensions up to 4. We then use a dimension witness to certify their dimensionality as well as their quantum nature. Our results open new avenues for the device-independent estimation of unknown quantum systems \cite{mayers, bardyn, bancal, rabelo} and for applications in quantum information science \cite{marcin, guo}.
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• Loophole-free quantum steering
Bernhard Wittmann, Sven Ramelow, Fabian Steinlechner, Nathan K. Langford, Nicolas Brunner, Howard Wiseman, Rupert Ursin, Anton Zeilinger
04 November 2011

Abstract:
Tests of the predictions of quantum mechanics for entangled systems have provided increasing evidence against local realistic theories 1-6. However, there still remains the crucial challenge of simultaneously closing all major loopholes - the locality, freedom-of-choice, and detection loopholes - in a single experiment. An important sub-class of local realistic theories can be tested with the concept of "steering". The term steering was introduced by Schr\"odinger in 1935 for the fact that entanglement would seem to allow an experimenter to remotely steer the state of a distant system 7. Einstein called this "spooky action at a distance" 8. Steering has recently been rigorously formulated as a quantum information task opening it up to new experimental tests 9-11. Here, we present the first loophole-free demonstration of steering by violating three-setting quadratic steering inequality, tested with polarization entangled photons shared between two distant laboratories. Our experiment demonstrates this effect while simultaneously closing all loopholes by a large separation, ultra-fast switching, quantum random number generation, and high, overall detection efficiency. Thereby, we exclude - for the first time loophole-free - an important class of local realistic theories. As well as its foundational importance 9,10, loop-hole-free steering also allows the secure distribution of quantum entanglement from an untrusted party 12.
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• Testing the Structure of Multipartite Entanglement with Bell Inequalities
Nicolas Brunner, James Sharam, Tamas Vertesi
26 October 2011

Abstract:
We show that the rich structure of multipartite entanglement can be tested following a device-independent approach. Specifically we present Bell inequalities that allow one to distinguish between different types of multipartite entanglement, without placing any further assumptions on the devices used in the protocol. We first address the case of three qubits and present Bell inequalities that can be violated by W states but not by GHZ states, and vice versa. Next, we devise 'sub-correlation Bell inequalities' for an arbitrary number of parties, which can provably not be violated by a broad class of multipartite entangled states (generalizations of GHZ states), but for which violations can be obtained for W states. Our results give insight into the nonlocality of W states. The simplicity and robustness of our tests make them appealing for experiments.
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• Increasing complexity with quantum physics
Janet Anders, Karoline Wiesner
25 October 2011

Abstract:
We argue that complex systems science and the rules of quantum physics are intricately related. We discuss a range of quantum phenomena, such as cryptography, computation and quantum phases, and the rules responsible for their complexity. We identify correlations as a central concept connecting quantum information and complex systems science. We present two examples for the power of correlations: using quantum resources to simulate the correlations of a stochastic process and to implement a classically impossible computational task.
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• Calculating Unknown Eigenvalues with a Quantum Algorithm
Xiao-Qi Zhou, Pruet Kalasuwan, Timothy C. Ralph, Jeremy L. O'Brien
20 October 2011

Abstract:
Quantum algorithms are able to solve particular problems exponentially faster than conventional algorithms, when implemented on a quantum computer. However, all demonstrations to date have required already knowing the answer to construct the algorithm. We have implemented the complete quantum phase estimation algorithm for a single qubit unitary in which the answer is calculated by the algorithm. We use a new approach to implementing the controlled-unitary operations that lie at the heart of the majority of quantum algorithms that is more efficient and does not require the eigenvalues of the unitary to be known. These results point the way to efficient quantum simulations and quantum metrology applications in the near term, and to factoring large numbers in the longer term. This approach is architecture independent and thus can be used in other physical implementations.
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• Can apparent superluminal neutrino speeds be explained as a quantum weak measurement?
M. V. Berry, N. Brunner, S. Popescu, P. Shukla
17 October 2011

Abstract:
Probably not.
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• Inverting the central limit theorem
Miguel Navascues, David Perez-Garcia, Ignacio Villanueva
12 October 2011

Abstract:
The central limit theorem states that the sum of N independently distributed n-tuples of real variables (subject to appropriate normalization) tends to a multivariate gaussian distribution for large N. Here we propose to invert this argument: given a set of n correlated gaussian variables, we try to infer information about the structure of the discrete microscopic probability distributions whose convolution generated such a macroscopic behavior. The techniques developed along the article are applied to prove that the classical description of certain macroscopic optical experiments is infinitely more complex than the quantum one.
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• Measuring protein concentration with entangled photons
Andrea Crespi, Mirko Lobino, Jonathan C. F. Matthews, Alberto Politi, Chris R. Neal, Roberta Ramponi, Roberto Osellame, Jeremy L. O'Brien
15 September 2011

Abstract:
Optical interferometry is amongst the most sensitive techniques for precision measurement. By increasing the light intensity a more precise measurement can usually be made. However, in some applications the sample is light sensitive. By using entangled states of light the same precision can be achieved with less exposure of the sample. This concept has been demonstrated in measurements of fixed, known optical components. Here we use two-photon entangled states to measure the concentration of the blood protein bovine serum albumin (BSA) in an aqueous buffer solution. We use an opto-fluidic device that couples a waveguide interferometer with a microfluidic channel. These results point the way to practical applications of quantum metrology to light sensitive samples.
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• Entanglement can completely defeat quantum noise
Jianxin Chen, Toby S. Cubitt, Aram W. Harrow, Graeme Smith
06 September 2011

Abstract:
We describe two quantum channels that individually cannot send any information, even classical, without some chance of decoding error. But together a single use of each channel can send quantum information perfectly reliably. This proves that the zero-error classical capacity exhibits superactivation, the extreme form of the superadditivity phenomenon in which entangled inputs allow communication over zero capacity channels. But our result is stronger still, as it even allows zero-error quantum communication when the two channels are combined. Thus our result shows a new remarkable way in which entanglement across two systems can be used to resist noise, in this case perfectly. We also show a new form of superactivation by entanglement shared between sender and receiver.
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• Generating, manipulating and measuring entanglement and mixture with a reconfigurable photonic circuit
P. J. Shadbolt, M. R. Verde, A. Peruzzo, A. Politi, A. Laing, M. Lobino, J. C. F. Matthews, J. L. O'Brien
17 August 2011

Abstract:
Entanglement is the quintessential quantum mechanical phenomenon understood to lie at the heart of future quantum technologies and the subject of fundamental scientific investigations. Mixture, resulting from noise, is often an unwanted result of interaction with an environment, but is also of fundamental interest, and is proposed to play a role in some biological processes. Here we report an integrated waveguide device that can generate and completely characterize pure two-photon states with any amount of entanglement and arbitrary single-photon states with any amount of mixture. The device consists of a reconfigurable integrated quantum photonic circuit with eight voltage controlled phase shifters. We demonstrate that for thousands of randomly chosen configurations the device performs with high fidelity. We generate maximally and non-maximally entangled states, violate a Bell-type inequality with a continuum of partially entangled states, and demonstrate generation of arbitrary one-qubit mixed states.
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• Fast path and polarisation manipulation of telecom wavelength single photons in lithium niobate waveguide devices
Damien Bonneau, Mirko Lobino, Pisu Jiang, Chandra M. Natarajan, Michael G. Tanner, Robert H. Hadfield, Sanders N. Dorenbos, Val Zwiller, Mark G. Thompson, Jeremy L. O'Brien
19 July 2011

Abstract:
We demonstrate fast polarisation and path control of photons at 1550 nm in lithium niobate waveguide devices using the electro-optic effect. We show heralded single photon state engineering, quantum interference, fast state preparation of two entangled photons and feedback control of quantum interference. These results point the way to a single platform that will enable the integration of nonlinear single photon sources and fast reconfigurable circuits for future photonic quantum information science and technology.
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• Heralded amplification for precision measurements with spin ensembles
Nicolas Brunner, Eugene S. Polzik, Christoph Simon
15 July 2011

Abstract:
We propose a simple heralded amplification scheme for small rotations of the collective spin of an ensemble of particles. Our protocol makes use of two basic primitives for quantum memories, namely partial mapping of light onto an ensemble, and conversion of a collective spin excitation into light. The proposed scheme should be realizable with current technology, with potential applications to atomic clocks and magnetometry.
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• Infinitely many constrained inequalities for the von Neumann entropy
Josh Cadney, Noah Linden, Andreas Winter
05 July 2011

Abstract:
We exhibit infinitely many new, constrained inequalities for the von Neumann entropy, and show that they are independent of each other and the known inequalities obeyed by the von Neumann entropy (basically strong subadditivity). The new inequalities were proved originally by Makarychev et al. [Commun. Inf. Syst., 2(2):147-166, 2002] for the Shannon entropy, using properties of probability distributions. Our approach extends the proof of the inequalities to the quantum domain, and includes their independence for the quantum and also the classical cases.
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• Disproving the Peres conjecture: nonlocality does not imply entanglement distillability
Tamas Vertesi, Nicolas Brunner
27 June 2011

Abstract:
Entanglement and nonlocality are both fundamental aspects of quantum mechanics. Moreover, they play a prominent role in quantum information science, where they represent powerful resources for information processing. The exact relation between entanglement and nonlocality is however still poorly understood. Here we make progress in this direction by showing that bound entanglement-the most contrived form of entanglement-can lead to nonlocality. Specifically, we present a 3-qubit fully bound entangled state-from which no pure bipartite entanglement can be distilled on any bipartition-and show that it violates a Bell inequality. This disproves a long-standing conjecture made by Peres in 1999, and shows that quantum nonlocality does not imply entanglement distillability.
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• Slow-light Enhanced Correlated Photon-Pair Generation in Silicon
C. Xiong, Christelle Monat, Alex S. Clark, Christian Grillet, Graham D. Marshall, M. J. Steel, Juntao Li, Liam O'Faolain, Thomas F. Krauss, John G. Rarity, Benjamin J. Eggleton
21 June 2011

Abstract:
We report the generation of correlated photon pairs in the telecom C-band, at room temperature, from a dispersion-engineered silicon photonic crystal waveguide. The spontaneous four-wave mixing process producing the photon pairs is enhanced by slow-light propagation enabling an active device length of less than 100 {\mu}m. With a coincidence to accidental ratio of 12.8, at a pair generation rate of 0.006 per pulse, this ultra-compact photon pair source is immediately applicable towards scalable quantum information processing realized on-chip.
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• From Qubits to Hyperbits
M. Pawlowski, A. Winter
14 June 2011

Abstract:
We propose to study hyperbits' as generalizations of quantum bits: their state spaces are d-dimensional Euclidean balls (d=3 is the Bloch sphere of qubits), while they can be subjected to binary measurements corresponding to any direction in d-dimensional Euclidean space. We show that sending and using one hyperbit from one player to another one is in a certain sense equivalent to sharing arbitrary entanglement and sending 1 classical bit -- at least in the context of certain non-local games. We also demonstrate a fundamental identity for hyperbits, limiting their information processing capabilities. As a consequence, we prove a stronger form of the Information Causality inequality for messages of 1 bit; it encapsulates not only the known information limitations of quantum theory but also complementarity.
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• Virtual qubits, virtual temperatures, and the foundations of thermodynamics
Nicolas Brunner, Noah Linden, Sandu Popescu, Paul Skrzypczyk
13 June 2011

Abstract:
We argue that thermal machines can be understood from the perspective of virtual qubits' at virtual temperatures': The relevant way to view the two heat baths which drive a thermal machine is as a composite system. Virtual qubits are two-level subsystems of this composite, and their virtual temperatures can take on any value, positive or negative. Thermal machines act upon an external system by placing it in thermal contact with a well-selected range of virtual qubits and temperatures. We demonstrate these claims by studying the smallest thermal machines. We show further that this perspective provides a powerful way to view thermodynamics, by analysing a number of phenomena. This includes approaching Carnot efficiency (where we find that all machines do so essentially by becoming equivalent to the smallest thermal machines), entropy production in irreversible machines, and a way to view work in terms of negative temperature and population inversion. Moreover we introduce the idea of "genuine" thermal machines and are led to considering the concept of "strength" of work.
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• Simulating quantum statistics with entangled photons: a continuous transition from bosons to fermions
Jonathan C. F. Matthews, Konstantinos Poulios, Jasmin D. A. Meinecke, Alberto Politi, Alberto Peruzzo, Nur Ismail, Kerstin Wörhoff, Mark G. Thompson, Jeremy L. O'Brien
07 June 2011

Abstract:
In contrast to classical physics, quantum mechanics divides particles into two classes-bosons and fermions-whose exchange statistics dictate the dynamics of systems at a fundamental level. In two dimensions quasi-particles known as 'anyons' exhibit fractional exchange statistics intermediate between these two classes. The ability to simulate and observe behaviour associated to fundamentally different quantum particles is important for simulating complex quantum systems. Here we use the symmetry and quantum correlations of entangled photons subjected to multiple copies of a quantum process to directly simulate quantum interference of fermions, bosons and a continuum of fractional behaviour exhibited by anyons. We observe an average similarity of 93.6\pm0.2% between an ideal model and experimental observation. The approach generalises to an arbitrary number of particles and is independent of the statistics of the particles used, indicating application with other quantum systems and large scale application.
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• Estimating Pre- and Post-Selected Ensembles
Serge Massar, Sandu Popescu
03 June 2011

Abstract:
In analogy with the usual state estimation problem, we introduce the problem of state estimation for a pre- and post-selected ensemble. The problem has fundamental physical significance since, as argued by Y. Aharonov and collaborators, pre- and post-selected ensembles are the most basic quantum ensembles. Two new features are shown to appear: 1) information is flowing to the measuring device both from the past and from the future; 2)because of the post-selection, certain measurement outcomes can be forced never to occur. Due to these features, state estimation in such ensembles is dramatically different from the case of ordinary, pre-selected only ensembles. Here we develop a general theoretical framework for studying this problem, and illustrate it through several examples. We also prove a general theorem showing that information flowing from the future is related to the complex conjugate information flowing from the past. We emphasize that {\it all} state estimation problems can be extended to the pre- and post-selected situation. The present work thus lays the foundations of a much more general theory of quantum state estimation.
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• Device-independent certification of entangled measurements
Rafael Rabelo, Melvyn Ho, Daniel Cavalcanti, Nicolas Brunner, Valerio Scarani
17 May 2011

Abstract:
We present a device-independent protocol to test if a given black-box measurement device is entangled, that is, has entangled eigenstates. Our scheme involves three parties and is inspired by entanglement swapping; the test uses the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality, checked between each pair of parties. Also, focusing on the case where all particles are qubits, we characterize quantitatively the deviation of the measurement device from a perfect Bell state measurement.
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• Quantum locking of classical correlations and quantum discord of classical-quantum states
S. Boixo, L. Aolita, D. Cavalcanti, K. Modi, A. Winter
16 May 2011

Abstract:
A locking protocol between two parties is as follows: Alice gives an encrypted classical message to Bob which she does not want Bob to be able to read until she gives him the key. If Alice is using classical resources, and she wants to approach unconditional security, then the key and the message must have comparable sizes. But if Alice prepares a quantum state, the size of the key can be comparatively negligible. This effect is called quantum locking. Entanglement does not play a role in this quantum advantage. We show that, in this scenario, the quantum discord quantifies the advantage of the quantum protocol over the corresponding classical one for any classical-quantum state.
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• Dynamical features of interference phenomena in the presence of entanglement
T. Kaufherr, Y. Aharonov, S. Nussinov, S. Popescu, J. Tollaksen
26 April 2011

Abstract:
A "strongly" interacting, and entangling, heavy, non recoiling, external particle effects a significant change of the environment. Described locally, the corresponding entanglement event is a generalized electric Aharonov Bohm effect, that differs from the original one in a crucial way. We propose a gedanken interference experiment. The predicted shift of the interference pattern is due to a self induced or "private" potential difference experienced while the particle is in vacuum. We show that all non trivial Born Oppenheimer potentials are "private" potentials. We apply the Born Oppenheimer approximation to interference states. Using our approach we calculate the relative phase of the external heavy particle as well as its uncertainty throughout an interference experiment /entanglement event. We thus complement the Born Oppenheimer approximation for interference states.
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• A critical view on transport and entanglement in models of photosynthesis
Markus Tiersch, Sandu Popescu, Hans J. Briegel
21 April 2011

Abstract:
Quantum effects in biological light-harvesting molecules, such as quantum coherence of excitonic states and entanglement have recently gained much attention. We observe a certain discrepancy between the original experimental work and several theoretical treatments of coherent excitation transport in light-harvesting molecules. Contrary to what is generally stated, we argue that entanglement in such molecules is generally not equivalent to the presence of coherence but mostly introduced by initial assumptions underlying the models, and that entanglement, as opposite to coherence, seems to play no role in the transport efficiency.
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• Communication cost of simulating entanglement swapping
Nicolas Brunner, Cyril Branciard, Nicolas Gisin, Denis Rosset
28 March 2011

Abstract:
Entanglement appears in two different ways in quantum mechanics, namely as a property of states and as a property of measurement outcomes in joint measurements. By combining these two aspects of entanglement, it is possible to generate nonlocality between particles that never interacted, using the protocol of entanglement swapping. We investigate the communication cost of classically simulating this process. While the communication cost of simulating nonlocal correlations of entangled states appears to be generally quite low, we show here that infinite communication is required to simulate entanglement swapping. This result is derived in the scenario of bilocality, where distant sources of particles are assumed to be independent, and takes advantage of a previous result of Massar et al. [Phys. Rev. A {\bf 63}, 052305 (2001)]. Our result implies that any classical model simulating entanglement swapping must either assume that (i) infinite shared randomness is available between any two locations in the universe, or that (ii) infinite communication takes place.
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• Semi-device-independent security of one-way quantum key distribution
Marcin Pawlowski, Nicolas Brunner
22 March 2011

Abstract:
By testing nonlocality, the security of entanglement-based quantum key distribution (QKD) can be enhanced to being 'device-independent'. Here we ask whether such a strong form of security could also be established for one-way QKD. While fully device-independent security is impossible, we show that security can be guaranteed against collective attacks in a semi-device-independent scenario. In the latter, the devices used by the trusted parties are non-characterized, but the dimensionality of the quantum systems used in the protocol is assumed to be bounded. Our security proof relies on the analogies between one-way QKD, dimension witnesses and random-access codes.
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• All non-classical correlations can be activated into distillable entanglement
Marco Piani, Sevag Gharibian, Gerardo Adesso, John Calsamiglia, Pawel Horodecki, Andreas Winter
22 March 2011

Abstract:
We devise a protocol in which general non-classical multipartite correlations produce a physically relevant effect, leading to the creation of bipartite entanglement. In particular, we show that the relative entropy of quantumness, which measures all non-classical correlations among subsystems of a quantum system, is equivalent to and can be operationally interpreted as the minimum distillable entanglement generated between the system and local ancillae in our protocol. We emphasize the key role of state mixedness in maximizing non-classicality: Mixed entangled states can be arbitrarily more non-classical than separable and pure entangled states.
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• Correlated photon-pair generation in a periodically poled MgO doped stoichiometric lithium tantalate reverse proton exchanged waveguide
M. Lobino, G. D. Marshall, C. Xiong, A. S. Clark, D. Bonneau, C. M. Natarajan, M. G. Tanner, R. H. Hadfield, S. N. Dorenbos, T. Zijlstra, V. Zwiller, M. Marangoni, R. Ramponi, M. G. Thompson, B. J. Eggleton, J. L. O'Brien
22 March 2011

Abstract:
We demonstrate photon-pair generation in a reverse proton exchanged waveguide fabricated on a periodically poled magnesium doped stoichiometric lithium tantalate substrate. Detected pairs are generated via a cascaded second order nonlinear process where a pump laser at wavelength of 1.55 $\mu$m is first doubled in frequency by second harmonic generation and subsequently downconverted around the same spectral region. Pairs are detected at a rate of 42 per second with a coincidence to accidental ratio of 0.7. This cascaded pair generation process is similar to four-wave-mixing where two pump photons annihilate and create a correlated photon pair.
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• The monomial representations of the Clifford group
D.M. Appleby, Ingemar Bengtsson, Stephen Brierley, David Gross, Jan-Ake Larsson
07 February 2011
arXiv:1102.1268 (2011)

Abstract:
We show that the Clifford group - the normaliser of the Weyl-Heisenberg group - can be represented by monomial phase-permutation matrices if and only if the dimension is a square number. This simplifies expressions for SIC vectors, and has other applications to SICs and to Mutually Unbiased Bases.
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• Limitations on quantum dimensionality reduction
Aram W. Harrow, Ashley Montanaro, Anthony J. Short
13 December 2010

Abstract:
The Johnson-Lindenstrauss Lemma is a classic result which implies that any set of n real vectors can be compressed to O(log n) dimensions while only distorting pairwise Euclidean distances by a constant factor. Here we consider potential extensions of this result to the compression of quantum states. We show that, by contrast with the classical case, there does not exist any distribution over quantum channels that significantly reduces the dimension of quantum states while preserving the 2-norm distance with high probability. We discuss two tasks for which the 2-norm distance is indeed the correct figure of merit. In the case of the trace norm, we show that the dimension of low-rank mixed states can be reduced by up to a square root, but that essentially no dimensionality reduction is possible for highly mixed states.
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• Large violation of Bell inequalities using both particle and wave measurements
Daniel Cavalcanti, Nicolas Brunner, Paul Skrzypczyk, Alejo Salles, Valerio Scarani
10 December 2010

Abstract:
When separated measurements on entangled quantum systems are performed, the theory pre- dicts correlations that cannot be explained by any classical mechanism: communication is excluded because the signal should travel faster than light; pre-established agreement is excluded because Bell inequalities are violated. All optical demonstrations of such violations have involved discrete degrees of freedom and are plagued by the detection-efficiency loophole. A promising alternative is to use continuous variables combined with highly efficient homodyne measurements. However, all the schemes proposed so far use states or measurements that are extremely difficult to achieve, or produce very weak violations. In this paper we show that large violations for feasible states can be achieved if both photon counting and homodyne detections are used. Our scheme may lead to the first violation of Bell inequalities using continuous-variable measurements and pave the way for a loophole-free Bell test.
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• Device-independent bounds on entanglement
Yeong-Cherng Liang, Tamas Vertesi, Nicolas Brunner
08 December 2010

Abstract:
Detection and quantification of entanglement in quantum resources are two key steps in the implementation of various quantum information processing tasks. Here, we show that Bell-type inequalities are not only useful in verifying the presence of entanglement, but can also be used to bound the entanglement of the underlying physical system. Our main tool consists of a family of Bell inequalities that cannot be violated maximally by any finite-dimensional maximally entangled state. The fact that these bounds arise from Bell-type inequalities also allow them to be obtained in a device-independent manner, i.e., without resorting to any knowledge of the actual measurements being performed on the individual subsystems.
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• Limits on non-local correlations from the structure of the local state space
Peter Janotta, Christian Gogolin, Jonathan Barrett, Nicolas Brunner
07 December 2010

Abstract:
We investigate the connection between the structure of local state spaces and the strength of nonlocal correlations in the context of generalized probabilistic theories. We first present a family of models where the local state spaces are given by regular polygons. For each model we define the analog of a maximally entangled state and characterize its nonlocal correlations. This allows us to study the transition between quantum correlations and general no-signaling correlations by modifying only the local state space. We find that the strength of nonlocal correlations depends crucially on a simple geometric property of the local state space, known as strong self-duality. Then we prove in general that the correlations of maximally entangled states in any strongly self-dual model are limited, since they must satisfy the principle of macroscopic locality. This implies notably that Tsirelson's bound for correlations of the maximally entangled state in quantum mechanics can be regarded as a consequence of strong self-duality of local quantum systems. Finally, our results also show that there exist models which are locally almost identical to quantum mechanics, but can nevertheless generate maximally nonlocal correlations.
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• Nano-fabricated solid immersion lenses registered to single emitters in diamond
L. Marseglia, J. P. Hadden, A. C. Stanley-Clarke, J. P. Harrison, B. Patton, Y.-L. D. Ho, B. Naydenov, F. Jelezko, J. Meijer, P. Dolan, J. M. Smith, J. G. Rarity, J. L. O'Brien
07 December 2010

Abstract:
We describe a technique for fabricating micro- and nano-structures incorporating fluorescent defects in diamond with a positional accuracy in the hundreds of nanometers. Using confocal fluorescence microscopy and focused ion beam (FIB) etching we first locate a suitable defect with respect to registration marks on the diamond surface and then etch a structure using these coordinates. We demonstrate the technique here by etching an 8 micron diameter hemisphere positioned such that a single negatively charged nitrogen-vacancy defect lies at its origin. This type of structure increases the photon collection efficiency by removing refraction and aberration losses at the diamond-air interface. We make a direct comparison of the fluorescence photon count rate before and after fabrication and observe an 8-fold increase due to the presence of the hemisphere.
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• Biased nonlocal quantum games
Thomas Lawson, Noah Linden, Sandu Popescu
30 November 2010

Abstract:
We address the question of when quantum entanglement is a useful resource for information processing tasks by presenting a new class of nonlocal games that are simple, direct, generalizations of the Clauser Horne Shimony Holt game. For some ranges of the parameters that specify the games, quantum mechanics offers an advantage, while, surprisingly, for others quantum mechanics is no more powerful than classical mechanics in performing the nonlocal task. This sheds new light on the difference between classical, quantum and super-quantum correlations.
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• Generation of correlated photon pairs in a chalcogenide As2S3 waveguide
C. Xiong, G. D. Marshall, A. Peruzzo, M. Lobino, A. S. Clark, D.-Y. Choi, S. J. Madden, C. M. Natarajan, M. G. Tanner, R. H. Hadfield, S. N. Dorenbos, T. Zijlstra, V. Zwiller, M. G. Thompson, J. G. Rarity, M. J. Steel, B. Luther-Davies, B. J. Eggleton, J. L. O'Brien
09 November 2010

Abstract:
We demonstrate the first 1550 nm correlated photon-pair source in an integrated glass platform-a chalcogenide As2S3 waveguide. A measured pair coincidence rate of 80 per second was achieved using 57 mW of continuous-wave pump. The coincidence to accidental ratio was shown to be limited by spontaneous Raman scattering effects that are expected to be mitigated by using a pulsed pump source.
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• Conditional phase shift from a quantum dot in a pillar microcavity
A.B. Young, R. Oulton, C.Y. Hu, A.C.T. Thijssen, C. Schneider, S. Reitzenstein, M. Kamp, S. Hoefling, L. Worschech, A. Forchel, J.G. Rarity
01 November 2010
Phys. Rev. A 84, 011803(R)(2011)

Abstract:
Large conditional phase shifts from coupled atom-cavity systems are a key requirement for building a spin photon interface. This in turn would allow the realisation of hybrid quantum information schemes using spin and photonic qubits. Here we perform high resolution reflection spectroscopy of a quantum dot resonantly coupled to a pillar microcavity. We show both the change in reflectivity as the quantum dot is tuned through the cavity resonance, and measure the conditional phase shift induced by the quantum dot using an ultra stable interferometer. These techniques could be extended to the study of charged quantum dots, where it would be possible to realise a spin photon interface.
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• Device-independent tests of classical and quantum dimensions
Rodrigo Gallego, Nicolas Brunner, Christopher Hadley, Antonio Acin
26 October 2010

Abstract:
We address the problem of testing the dimensionality of classical and quantum systems in a black-box' scenario. We develop a general formalism for tackling this problem. This allows us to derive lower bounds on the classical dimension necessary to reproduce given measurement data. Furthermore, we generalise the concept of quantum dimension witnesses to arbitrary quantum systems, allowing one to place a lower bound on the Hilbert space dimension necessary to reproduce certain data. Illustrating these ideas, we provide simple examples of classical and quantum dimension witnesses.
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• Correlation and Entanglement of Multipartite States
Y. B. Band, I. Osherov
26 October 2010

Abstract:
We derive a classification and a measure of classical- and quantum-correlation of multipartite qubit, qutrit, and in general, $n$-level systems, in terms of SU$(n)$ representations of density matrices. We compare the measure for the case of bipartite correlation with concurrence and the entropy of entanglement. The characterization of correlation is in terms of the number of nonzero singular values of the correlation matrix, but that of mixed state entanglement requires additional invariant parameters in the density matrix. For the bipartite qubit case, the condition for mixed state entanglement is written explicitly in terms of the invariant paramters in the density matrix. For identical particle systems we analyze the effects of exchange symmetry on classical and quantum correlation.
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• (Non-)Contextuality of Physical Theories as an Axiom
Adan Cabello, Simone Severini, Andreas Winter
12 October 2010

Abstract:
We show that the noncontextual inequality proposed by Klyachko et al. [Phys. Rev. Lett. 101, 020403 (2008)] belongs to a broader family of inequalities, one associated to each compatibility structure of a set of events (a graph), and its independence number. These have the surprising property that the maximum quantum violation is given by the Lovasz theta-function of the graph, which was originally proposed as an upper bound on its Shannon capacity. Furthermore, probabilistic theories beyond quantum mechanics may have an even larger violation, which is given by the so-called fractional packing number. We discuss in detail, and compare, the sets of probability distributions attainable by noncontextual, quantum, and generalized models; the latter two are shown to have semidefinite and linear characterizations, respectively. The implications for Bell inequalities, which are examples of noncontextual inequalities, are discussed. In particular, we show that every Bell inequality can be recast as a noncontextual inequality a la Klyachko et al.
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• Minimal and Maximal Operator Spaces and Operator Systems in Entanglement Theory
Nathaniel Johnston, David W. Kribs, Vern I. Paulsen, Rajesh Pereira
08 October 2010

Abstract:
We examine k-minimal and k-maximal operator spaces and operator systems, and investigate their relationships with the separability problem in quantum information theory. We show that the matrix norms that define the k-minimal operator spaces are equal to a family of norms that have been studied independently as a tool for detecting k-positive linear maps and bound entanglement. Similarly, we investigate the k-super minimal and k-super maximal operator systems that were recently introduced and show that their cones of positive elements are exactly the cones of k-block positive operators and (unnormalized) states with Schmidt number no greater than k, respectively. We characterize a class of norms on the k-super minimal operator systems and show that the completely bounded versions of these norms provide a criterion for testing the Schmidt number of a quantum state that generalizes the recently-developed separability criterion based on trace-contractive maps.
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• Tight Bounds for Classical and Quantum Coin Flipping
Esther Hänggi, Jürg Wullschleger
27 September 2010

Abstract:
Coin flipping is a cryptographic primitive for which strictly better protocols exist if the players are not only allowed to exchange classical, but also quantum messages. During the past few years, several results have appeared which give a tight bound on the range of implementable unconditionally secure coin flips, both in the classical as well as in the quantum setting and for both weak as well as strong coin flipping. However, all these results consider only protocols with perfect correctness, i.e., where two honest players must always output the same value and never abort. We remove this restriction by giving a more general definition of coin flipping which unifies the notion of strong and weak coin flipping (it contains both of them as special cases) and allows the honest players to abort with a certain probability. We give tight bounds on the achievable range of parameters both in the classical and in the quantum setting.
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• Bound non-locality and activation
Nicolas Brunner, Daniel Cavalcanti, Alejo Salles, Paul Skrzypczyk
23 September 2010

Abstract:
We investigate non-locality distillation using measures of non-locality based on the Elitzur-Popescu-Rohrlich decomposition. For a certain number of copies of a given non-local correlation, we define two quantities of interest: (i) the non-local cost, and (ii) the distillable non-locality. We find that there exist correlations whose distillable non-locality is strictly smaller than their non-local cost. Thus non-locality displays a form of irreversibility which we term bound non-locality. Finally we show that non-local distillability can be activated.
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• Maximally efficient quantum thermal machines: The basic principles
Sandu Popescu
15 September 2010

Abstract:
Following the result by Skrzypczyk et al., arXiv:1009.0865, that certain self-contained quantum thermal machines can reach Carnot efficiency, we discuss the functioning of self-contained quantum thermal machines and show, in a very general case, that they can reach the Carnot efficiency limit. Most importantly, the full analytical solution for the functioning of the machines is not required; the efficiency can be deduced from a very small number of fundamental and highly intuitive equations which capture the core of the problem.
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• Absence of thermalization in non-integrable systems
Christian Gogolin, Markus P. Mueller, Jens Eisert
14 September 2010

Abstract:
We present results establishing a link between unitary relaxation dynamics after a quench in closed many-body systems in non-equilibrium and the entanglement in the energy eigenbasis. We find that even if reduced states equilibrate, and appear perfectly relaxed, they can still have memory on the initial conditions even in models that are far from integrable, thereby giving rise to "equilibration without thermalization". We show that in such situations the equilibrium states are, however, still described by a Jaynes maximum entropy or generalized Gibbs ensemble and, moreover, that this is always the case if equilibration happens, regardless of whether a model is integrable or not. In addition, we discuss individual aspects of thermalization processes separately, comment on the role of Anderson localization, and collect and compare different notions of integrability.
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• Coherence properties of a single dipole emitter in diamond
Graham D. Marshall, Torsten Gaebel, Jonathan C. F. Matthews, Jorg Enderlein, Jeremy L. O'Brien, James R. Rabeau
14 September 2010

Abstract:
On-demand, high repetition rate sources of indistinguishable, polarised single photons are the key component for future photonic quantum technologies. Colour centres in diamond offer a promising solution, and the narrow line-width of the recently identified nickel-based NE8 centre makes it particularly appealing for realising the transform-limited sources necessary for quantum interference. Here we report the characterisation of dipole orientation and coherence properties of a single NE8 colour centre in a diamond nanocrystal at room-temperature. We observe a single photon coherence time of 0.21 ps and an emission lifetime of 1.5 ns. Combined with an emission wavelength that is ideally suited for applications in existing quantum optical systems, these results show that the NE8 is a far more promising source than the more commonly studied nitrogen-vacancy centre and point the way to the realisation of a practical diamond colour centre-based single photon source.
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• Quantum Computing
Thaddeus D. Ladd, Fedor Jelezko, Raymond Laflamme, Yasunobu Nakamura, Christopher Monroe, Jeremy L. O'Brien
14 September 2010

Abstract:
Quantum mechanics---the theory describing the fundamental workings of nature---is famously counterintuitive: it predicts that a particle can be in two places at the same time, and that two remote particles can be inextricably and instantaneously linked. These predictions have been the topic of intense metaphysical debate ever since the theory's inception early last century. However, supreme predictive power combined with direct experimental observation of some of these unusual phenomena leave little doubt as to its fundamental correctness. In fact, without quantum mechanics we could not explain the workings of a laser, nor indeed how a fridge magnet operates. Over the last several decades quantum information science has emerged to seek answers to the question: can we gain some advantage by storing, transmitting and processing information encoded in systems that exhibit these unique quantum properties? Today it is understood that the answer is yes. Many research groups around the world are working towards one of the most ambitious goals humankind has ever embarked upon: a quantum computer that promises to exponentially improve computational power for particular tasks. A number of physical systems, spanning much of modern physics, are being developed for this task---ranging from single particles of light to superconducting circuits---and it is not yet clear which, if any, will ultimately prove successful. Here we describe the latest developments for each of the leading approaches and explain what the major challenges are for the future.
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• On the efficiency of very small refrigerators
Paul Skrzypczyk, Nicolas Brunner, Noah Linden, Sandu Popescu
07 September 2010

Abstract:
We investigate whether size imposes a fundamental constraint on the efficiency of small thermal machines. We analyse in detail a model of a small self-contained refrigerator consisting of three qubits. We show that this system can reach the Carnot efficiency, and thus demonstrate that there exists no complementarity between size and efficiency.
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• Coherent Control of Rydberg States in Silicon
P. T. Greenland, S. A. Lynch, A. F. G. van der Meer, B. N. Murdin, C. R. Pidgeon, B. Redlich, N. Q. Vinh, G. Aeppli
24 August 2010

Abstract:
We demonstrate coherent control of donor wavefunctions in phosphorous-doped silicon. Our experiments take advantage of a free electron laser to stimulate and observe photon echoes from, and Rabi oscillations between the ground and first excited state of P donors in Si.
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• Characterizing Operations Preserving Separability Measures via Linear Preserver Problems
Nathaniel Johnston
24 August 2010

Abstract:
We use classical results from the theory of linear preserver problems to characterize operators that send the set of pure states with Schmidt rank no greater than k back into itself, extending known results characterizing operators that send separable pure states to separable pure states. We also prove an analogous statement in the multipartite setting. We use this result to develop a bipartite version of a classical result about the structure of maps that preserve rank-1 operators and then characterize the isometries for two families of norms that have recently been studied in quantum information theory. We see in particular that for k at least 2, the operator norms induced by states with Schmidt rank k are invariant only under local unitaries, the swap operator and the transpose map. However, in the k = 1 case there is an additional isometry: the partial transpose map.
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• How much free will is needed to demonstrate nonlocality?
Jonathan Barrett, Nicolas Gisin
24 August 2010

Abstract:
If nonlocality is to be inferred from a violation of Bell's inequality, an important assumption is that the measurement settings are freely chosen by the observers, or alternatively, that they are random and uncorrelated with the hypothetical local variables. We study the case where this assumption is weakened, so that measurement settings and local variables can be at least partially correlated. We demonstrate a connection between this type of model and classical communication models, and a connection with models that exploit the detection efficiency loophole. We show that even if Bob enjoys full free will, if Alice lacks a single bit of free will - in the sense that the mutual information between local variables and her measurement setting is one bit - then all correlations obtained from projective measurements on a singlet can be reproduced by local means.
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• Operational interpretations of quantum discord
D. Cavalcanti, L. Aolita, S. Boixo, K. Modi, M. Piani, A. Winter
20 August 2010

Abstract:
Quantum discord is a quantifier of non-classical correlations that goes beyond the standard classification of quantum states into entangled and unentangled ones. Although it has received considerable attention, it still lacks any precise interpretation in terms of some protocol in which quantum features are relevant. Here we give quantum discord its first operational meaning in terms on consumption of entanglement in an extended quantum state merging protocol. We go on to show that the asymmetry of quantum discord is related to the performance imbalance in quantum state merging and dense coding.
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• A new exponential separation between quantum and classical one-way communication complexity
Ashley Montanaro
22 July 2010

Abstract:
We present a new example of a partial boolean function whose one-way quantum communication complexity is exponentially lower than its one-way classical communication complexity. The problem is a natural generalisation of the previously studied Subgroup Membership problem: Alice receives a bit string x, Bob receives a permutation matrix M, and their task is to determine whether Mx=x or Mx is far from x. The proof uses Fourier analysis, an inequality of Kahn, Kalai and Linial, and bounds on the Krawtchouk polynomials.
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• Physics within a quantum reference frame
Renato M. Angelo, Nicolas Brunner, Sandu Popescu, Antony J. Short, Paul Skrzypczyk
15 July 2010

Abstract:
We investigate the physics of quantum reference frames. Specifically, we study several simple scenarios involving a small number of quantum particles, whereby we promote one of these particles to the role of a quantum observer and ask what is the description of the rest of the system, as seen by this observer? We highlight the interesting aspects of such questions by presenting a number of apparent paradoxes. By unravelling these paradoxes we get a better understanding of the physics of quantum reference frames.
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• Multimode quantum interference of photons in multiport integrated devices
Alberto Peruzzo, Anthony Laing, Alberto Politi, Terry Rudolph, Jeremy L. O'Brien
12 July 2010

Abstract:
We report the first demonstration of quantum interference in multimode interference (MMI) devices and a new complete characterization technique that can be applied to any photonic device that removes the need for phase stable measurements. MMI devices provide a compact and robust realization of NxM optical circuits, which will dramatically reduce the complexity and increase the functionality of future generations of quantum photonic circuits.
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• Quantum walks of correlated particles
Alberto Peruzzo, Mirko Lobino, Jonathan C. F. Matthews, Nobuyuki Matsuda, Alberto Politi, Konstantinos Poulios, Xiao-Qi Zhou, Yoav Lahini, Nur Ismail, Kerstin Wörhoff, Yaron Bromberg, Yaron Silberberg, Mark G. Thompson, Jeremy L. O'Brien
25 June 2010

Abstract:
Quantum walks of correlated particles offer the possibility to study large-scale quantum interference, simulate biological, chemical and physical systems, and a route to universal quantum computation. Here we demonstrate quantum walks of two identical photons in an array of 21 continuously evanescently-coupled waveguides in a SiOxNy chip. We observe quantum correlations, violating a classical limit by 76 standard deviations, and find that they depend critically on the input state of the quantum walk. These results open the way to a powerful approach to quantum walks using correlated particles to encode information in an exponentially larger state space.
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• Realization of a Knill-Laflamme-Milburn C-NOT gate -a photonic quantum circuit combining effective optical nonlinearities
Ryo Okamoto, Jeremy L. O'Brien, Holger F. Hofmann, Shigeki Takeuchi
25 June 2010

Abstract:
Quantum information science addresses how uniquely quantum mechanical phenomena such as superposition and entanglement can enhance communication, information processing and precision measurement. Photons are appealing for their low noise, light-speed transmission and ease of manipulation using conventional optical components. However, the lack of highly efficient optical Kerr nonlinearities at single photon level was a major obstacle. In a breakthrough, Knill, Laflamme and Milburn (KLM) showed that such an efficient nonlinearity can be achieved using only linear optical elements, auxiliary photons, and measurement. They proposed a heralded controlled-NOT (CNOT) gate for scalable quantum computation using a photonic quantum circuit to combine two such nonlinear elements. Here we experimentally demonstrate a KLM CNOT gate. We developed a stable architecture to realize the required four-photon network of nested multiple interferometers based on a displaced-Sagnac interferometer and several partially polarizing beamsplitters. This result confirms the first step in the KLM recipe' for all-optical quantum computation, and should be useful for on-demand entanglement generation and purification. Optical quantum circuits combining giant optical nonlinearities may find wide applications across telecommunications and sensing.
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• Adding control to arbitrary quantum operations
Xiao-Qi Zhou, Timothy C. Ralph, Pruet Kalasuwan, Mian Zhang, Alberto Peruzzo, Benjamin P. Lanyon, Jeremy L. O'Brien
15 June 2010

Abstract:
Quantum computers promise exponential power for particular tasks, however, the complexity of quantum algorithms remains a major technological challenge. We have developed and demonstrated an architecture independent technique for adding control qubits to arbitrary quantum operations (unitary or otherwise) - a key requirement in many quantum algorithms. The technique is independent of how the operation is done and does not even require knowledge of what the operation is. In this way the technical problems of how to implement a quantum operation and how to add a control are separated. The number of computational resources required is independent of the depth of the operation and increases only linearly with the number of qubits on which it acts. Our approach will significantly reduce the complexity of quantum computations such as Shor's factoring algorithm and the near-term prospect of quantum simulations. We use this new approach to implement a number of two-qubit photonic quantum gates in which the operation of the control circuit is completed independent of the choice of quantum operation.
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• Strongly enhanced photon collection from diamond colour centres under micro-fabricated integrated solid immersion lenses
J. P. Hadden, J. P. Harrison, A. C. Stanley-Clarke, L. Marseglia, Y-L. D. Ho, B. R. Patton, J. L. O'Brien, J. G. Rarity
12 June 2010

Abstract:
The efficiency of collecting photons from optically active defect centres in bulk diamond is greatly reduced by refraction and reflection at the diamond-air interface. We report on the fabrication and measurement of a geometrical solution to the problem; integrated solid immersion lenses (SILs) etched directly into the surface of diamond. An increase of a factor of 10 was observed in the saturated count-rate from a single negatively charged nitrogen-vacancy (NV-) within a 5um diameter SIL compared with NV-s under a planar surface in the same crystal. A factor of 3 reduction in background emission was also observed due to the reduced excitation volume with a SIL present. Such a system is potentially scalable and easily adaptable to other defect centres in bulk diamond.
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• Mutually Unbiased Bases and Semi-definite Programming
Stephen Brierley, Stefan Weigert
01 June 2010
J. Phys.: Conf. Ser. 254 012008 (2010)

Abstract:
A complex Hilbert space of dimension six supports at least three but not more than seven mutually unbiased bases. Two computer-aided analytical methods to tighten these bounds are reviewed, based on a discretization of parameter space and on Grobner bases. A third algorithmic approach is presented: the non-existence of more than three mutually unbiased bases in composite dimensions can be decided by a global optimization method known as semidefinite programming. The method is used to confirm that the spectral matrix cannot be part of a complete set of seven mutually unbiased bases in dimension six.
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• Loss-resistant state teleportation and entanglement swapping using a quantum-dot spin in an optical microcavity
C.Y. Hu, J.G. Rarity
30 May 2010
Phys. Rev. B 83, 115303 (2011)

Abstract:
We present a scheme for deterministic state teleportation and entanglement swapping using a single quantum-dot spin in an optical microcavity based on giant circular birefringence. State teleportation or entanglement swapping is heralded by the sequential detection of two photons, and is finished after the spin measurement. The spin-cavity unit works as a complete deterministic Bell-state analyzer with a built-in spin memory allowing loss-resistant repeater operation, and generally it is an arbitrary entanglement generator and analyzer. This scheme can be extended to quantum teleportation based on multi-particle entanglement.
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• Heralded entanglement for quantum enhanced measurement with photons
Jonathan C. F. Matthews, Alberto Politi, Damien Bonneau, Jeremy L. O'Brien
28 May 2010

Abstract:
Generating quantum entanglement is not only an important scientific endeavour, but will be essential to realising the tremendous potential of quantum-enhanced technologies, in particular quantum-enhanced measurements with precision beyond classical limits. We report the heralded generation of multi-photon entanglement for quantum metrology using a reconfigurable integrated waveguide device in which projective measurement of auxiliary photons heralds the generation of path entangled states. From four and six photon inputs we heralded two- and four-photon"NOON" states-a superposition of N photons in two paths, which enable phase supersensitive measurements at the Heisenberg limit. Realistic devices will include imperfections and we demonstrate phase super- resolution with a state that is robust to photon loss. These results can be generalised to generate arbitrarily large entangled states of light for quantum metrology in an integrated optics architecture.
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• Binary Matroids and Quantum Probability Distributions
Dan Shepherd
12 May 2010

Abstract:
We characterise the probability distributions that arise from quantum circuits all of whose gates commute, and show when these distributions can be classically simulated efficiently. We consider also marginal distributions and the computation of correlation coefficients, and draw connections between the simulation of stabiliser circuits and the combinatorics of representable matroids, as developed in the 1990s.
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• Quantum Complexity: restrictions on algorithms and architectures
Daniel James Shepherd
11 May 2010

Abstract:
A dissertation submitted to the University of Bristol in accordance with the requirements of the degree of Doctor of Philosophy (PhD) in the Faculty of Engineering, Department of Computer Science, July 2009.
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• Reduce, reuse, recycle, for robust cluster state generation
Clare Horsman, Katherine L. Brown, William J. Munro, Vivien M. Kendon
11 May 2010

Abstract:
Efficient generation of cluster states is crucial for engineering large-scale measurement-based quantum computers. Hybrid matter-optical systems offer a robust, scalable path to this goal. Such systems have an ancilla which acts as a bus connecting the qubits. We show that by generating smaller cluster "Lego blocks", reusing one ancilla per block, the cluster can be produced with maximal efficiency, requiring less than half the operations compared with no bus reuse. Our results are general for all ancilla-based computational schemes; we describe it in detail for the qubus system. By reducing the time required to prepare sections of the cluster, bus reuse more than doubles the size of the computational workspace that can be used before decoherence effects dominate.
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• Classical simulation of commuting quantum computations implies collapse of the polynomial hierarchy
Michael J. Bremner, Richard Jozsa, Dan J. Shepherd
11 May 2010

Abstract:
We consider quantum computations comprising only commuting gates, known as IQP computations, and provide compelling evidence that the task of sampling their output probability distributions is unlikely to be achievable by any efficient classical means. More specifically we introduce the class post-IQP of languages decided with bounded error by uniform families of IQP circuits with post-selection, and prove first that post-IQP equals the classical class PP. Using this result we show that if the output distributions of uniform IQP circuit families could be classically efficiently sampled, even up to 41% multiplicative error in the probabilities, then the infinite tower of classical complexity classes known as the polynomial hierarchy, would collapse to its third level. We mention some further results on the classical simulation properties of IQP circuit families, in particular showing that if the output distribution results from measurements on only O(log n) lines then it may in fact be classically efficiently sampled.
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• Determining dynamical equations is hard
Toby S. Cubitt, Jens Eisert, Michael M. Wolf
04 May 2010

Abstract:
The behaviour of any physical system is governed by its underlying dynamical equations--the differential equations describing how the system evolves with time--and much of physics is ultimately concerned with discovering these dynamical equations and understanding their consequences. At the end of the day, any such dynamical law is identified by making measurements at different times, and computing the dynamical equation consistent with the acquired data. In this work, we show that, remarkably, this process is a provably computationally intractable problem (technically, it is NP-hard). That is, even for a moderately complex system, no matter how accurately we have specified the data, discovering its dynamical equations can take an infeasibly long time (unless P=NP). As such, we find a complexity-theoretic solution to both the quantum and the classical embedding problems; the classical version is a long-standing open problem, dating from 1937, which we finally lay to rest.
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• Entanglement consumption of instantaneous nonlocal quantum measurements
S.R. Clark, A.J. Connor, D. Jaksch, S. Popescu
07 April 2010

Abstract:
Relativistic causality has dramatic consequences on the measurability of nonlocal variables and poses the fundamental question of whether it is physically meaningful to speak about the value of nonlocal variables at a particular time. Recent work has shown that by weakening the role of the measurement in preparing eigenstates of the variable it is in fact possible to measure all nonlocal observables instantaneously by exploiting entanglement. However, for these measurement schemes to succeed with certainty an infinite amount of entanglement must be distributed initially and all this entanglement is necessarily consumed. In this work we sharpen the characterisation of instantaneous nonlocal measurements by explicitly devising schemes in which only a finite amount of the initially distributed entanglement is ever utilised. This enables us to determine an upper bound to the average consumption for the most general cases of nonlocal measurements. This includes the tasks of state verification, where the measurement verifies if the system is in a given state, and verification measurements of a general set of eigenstates of an observable. Despite its finiteness the growth of entanglement consumption is found to display an extremely unfavourable exponential of an exponential scaling with either the number of qubits needed to contain the Schmidt rank of the target state or total number of qubits in the system for an operator measurement. This scaling is seen to be a consequence of the combination of the generic exponential scaling of unitary decompositions combined with the highly recursive structure of our scheme required to overcome the no-signalling constraints of relativistic causality.
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• High-fidelity operation of quantum photonic circuits
Anthony Laing, Alberto Peruzzo, Alberto Politi, Maria Rodas Verde, Matthaeus Halder, Timothy C. Ralph, Mark G. Thompson, Jeremy L. Oâ€™Brien
05 April 2010

Abstract:
We demonstrate photonic quantum circuits that operate at the stringent levels that will be required for future quantum information science and technology. These circuits are fabricated from silica-on-silicon waveguides forming directional couplers and interferometers. While our focus is on the operation of quantum circuits, to test this operation required construction of a spectrally tuned photon source to produce near-identical pairs of photons. We show non-classical interference with two photons and a two-photon entangling logic gate that operate with near-unit fidelity. These results are a significant step towards large-scale operation of photonic quantum circuits.
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• Operating quantum waveguide circuits with superconducting single-photon detectors
C. M. Natarajan, A. Peruzzo, S. Miki, M. Sasaki, Z. Wang, B. Baek, S. Nam, R. H. Hadfield, J. L. O'Brien
25 March 2010
Appl. Phys. Lett. 96, 211101 (2010)

Abstract:
Advanced quantum information science and technology (QIST) applications place exacting de- mands on optical components. Quantum waveguide circuits offer a route to scalable QIST on a chip. Superconducting single-photon detectors (SSPDs) provide infrared single-photon sensitivity combined with low dark counts and picosecond timing resolution. In this study we bring these two technologies together. Using SSPDs we observe a two-photon interference visibility of 92.3\pm1.0% in a silica-on-silicon waveguide directional coupler at \lamda = 804 nm-higher than that measured with silicon detectors (89.9\pm0.3%). We further operated controlled-NOT gate and quantum metrology circuits with SSPDs. These demonstrations present a clear path to telecom-wavelength quantum waveguide circuits.
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• A simple scheme for expanding photonic cluster states for quantum information
Pruet Kalasuwan, Gabriel Mendoza, Anthony Laing, Tomohisa Nagata, Jack Coggins, Mark Callaway, Shigeki Takeuchi, Andre Stefanov, Jeremy L. O'Brien
24 March 2010
JOSA B, Vol. 27, Issue 6, pp. A181-A184 (2010)

Abstract:
We show how an entangled cluster state encoded in the polarization of single photons can be straightforwardly expanded by deterministically entangling additional qubits encoded in the path degree of freedom of the constituent photons. This can be achieved using a polarization--path controlled-phase gate. We experimentally demonstrate a practical and stable realization of this approach by using a Sagnac interferometer to entangle a path qubit and polarization qubit on a single photon. We demonstrate precise control over phase of the path qubit to change the measurement basis and experimentally demonstrate properties of measurement-based quantum computing using a 2 photon, 3 qubit cluster state.
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• Photonic quantum technologies
Jeremy L. O'Brien, Akira Furusawa, Jelena Vučković
23 March 2010
Nature Photonics 3, 687 (2009)

Abstract:
The first quantum technology, which harnesses uniquely quantum mechanical effects for its core operation, has arrived in the form of commercially available quantum key distribution systems that achieve enhanced security by encoding information in photons such that information gained by an eavesdropper can be detected. Anticipated future quantum technologies include large-scale secure networks, enhanced measurement and lithography, and quantum information processors, promising exponentially greater computation power for particular tasks. Photonics is destined for a central role in such technologies owing to the need for high-speed transmission and the outstanding low-noise properties of photons. These technologies may use single photons or quantum states of bright laser beams, or both, and will undoubtably apply and drive state-of-the-art developments in photonics.
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• Guess your neighbour's input: a multipartite non-local game with no quantum advantage
M. L. Almeida, J.-D. Bancal, N. Brunner, A. Acin, N. Gisin, S. Pironio
22 March 2010
Phys. Rev. Lett. 104, 230404 (2010)

Abstract:
We present a multipartite nonlocal game in which each player must guess the input received by his neighbour. We show that quantum correlations do not perform better than classical ones at this game, for any prior distribution of the inputs. There exist, however, input distributions for which general no-signalling correlations can outperform classical and quantum correlations. Some of the Bell inequalities associated to our construction correspond to facets of the local polytope. Thus our multipartite game identifies parts of the boundary between quantum and post-quantum correlations of maximal dimension. These results suggest that quantum correlations might obey a generalization of the usual no-signalling conditions in a multipartite setting.
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• Zero-error channel capacity and simulation assisted by non-local correlations
Toby S. Cubitt, Debbie Leung, William Matthews, Andreas Winter
17 March 2010

Abstract:
Shannon's theory of zero-error communication is re-examined in the broader setting of using one classical channel to simulate another exactly, and in the presence of various resources that are all classes of non-signalling correlations: Shared randomness, shared entanglement and arbitrary non-signalling correlations. Specifically, when the channel being simulated is noiseless, this reduces to the zero-error capacity of the channel, assisted by the various classes of non-signalling correlations. When the resource channel is noiseless, it results in the "reverse" problem of simulating a noisy channel exactly by a noiseless one, assisted by correlations. In both cases, 'one-shot' separations between the power of the different assisting correlations are exhibited. The most striking result of this kind is that entanglement can assist in zero-error communication, in stark contrast to the standard setting of communicaton with asymptotically vanishing error in which entanglement does not help at all. In the asymptotic case, shared randomness is shown to be just as powerful as arbitrary non-signalling correlations for noisy channel simulation, which is not true for the asymptotic zero-error capacities. For assistance by arbitrary non-signalling correlations, linear programming formulas for capacity and simulation are derived, the former being equal (for channels with non-zero unassisted capacity) to the feedback-assisted zero-error capacity originally derived by Shannon to upper bound the unassisted zero-error capacity. Finally, a kind of reversibility between non-signalling-assisted capacity and simulation is observed, mirroring the famous "reverse Shannon theorem".
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• Tensor Rank and Stochastic Entanglement Catalysis for Multipartite Pure States
Lin Chen, Eric Chitambar, Runyao Duan, Zhengfeng Ji, Andreas Winter
17 March 2010

Abstract:
The tensor rank (aka generalized Schmidt rank) of multipartite pure states plays an important role in the study of entanglement classifications and transformations. We employ powerful tools from the theory of homogeneous polynomials to investigate the tensor rank of symmetric states such as the tripartite state $\ket{W_3}=\tfrac{1}{\sqrt{3}}(\ket{100}+\ket{010}+\ket{001})$ and its $N$-partite generalization $\ket{W_N}$. Previous tensor rank estimates are dramatically improved and we show that (i) three copies of $\ket{W_3}$ has rank either 15 or 16, (ii) two copies of $\ket{W_N}$ has rank $3N-2$, and (iii) $n$ copies of $\ket{W_N}$ has rank O(N). A remarkable consequence of these results is that certain multipartite transformations, impossible even probabilistically, can become possible when performed in multiple copy bunches or when assisted by some catalyzing state. This novel effect is impossible for bipartite pure states.
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• Quantum Memories. A Review based on the European Integrated Project "Qubit Applications (QAP)"
C. Simon, M. Afzelius, J. Appel, A. Boyer de la Giroday, S.J. Dewhurst, N. Gisin, C.Y. Hu, F. Jelezko, S. Kroll, J.H. Muller, J. Nunn, E. Polzik, J. Rarity, H. de Riedmatten, W. Rosenfeld, A.J. Shields, N. Skold, R.M. Stevenson, R. Thew, I. Walmsley, M. Weber, H. Weinfurter, J. Wrachtrup, R.J. Young
05 March 2010

Abstract:
We perform a review of various approaches to the implementation of quantum memories, with an emphasis on activities within the quantum memory sub-project of the EU Integrated Project "Qubit Applications". We begin with a brief overview over different applications for quantum memories and different types of quantum memories. We discuss the most important criteria for assessing quantum memory performance and the most important physical requirements. Then we review the different approaches represented in "Qubit Applications" in some detail. They include solid-state atomic ensembles, NV centers, quantum dots, single atoms, atomic gases and optical phonons in diamond. We compare the different approaches using the discussed criteria.
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• Reference frame independent quantum key distribution
Anthony Laing, Valerio Scarani, John G. Rarity, Jeremy L. O'Brien
05 March 2010

Abstract:
We describe a quantum key distribution protocol based on pairs of entangled qubits that generates a secure key between two partners in an environment of unknown and slowly varying reference frame. A direction of particle delivery is required, but the phases between the computational basis states need not be known or fixed. The protocol can simplify the operation of existing setups and has immediate applications to emerging scenarios such as earth-to-satellite links and the use of integrated photonic waveguides. We compute the asymptotic secret key rate for a two-qubit source, which coincides with the rate of the six-state protocol for white noise. We give the generalization of the protocol to higher-dimensional systems and detail a scheme for physical implementation in the three dimensional qutrit case.
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• Zero-error communication via quantum channels, non-commutative graphs and a quantum Lovasz theta function
Runyao Duan, Simone Severini, Andreas Winter
15 February 2010

Abstract:
We study the quantum channel version of Shannon's zero-error capacity problem. Motivated by recent progress on this question, we propose to consider a certain operator space as the quantum generalisation of the adjacency matrix, in terms of which the plain, quantum and entanglement-assisted capacity can be formulated.
Most importantly, we define a quantum version of Lovasz' famous theta function, as the norm-completion (or stabilisation) of a "naive" generalisation of theta. We go on to show that this function upper bounds the number of entanglement-assisted zero-error messages, that it is given by a semidefinite programme, whose dual we write down explicitly, and that it is multiplicative with respect to the natural graph product.
We explore various other properties of the new quantity, which reduces to Lovasz' original theta in the classical case, give several applications, and propose to study the operator spaces associated to channels as "non-commutative graphs".
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• Hidden Quantum Markov Models and non-adaptive read-out of many-body states
Alex Monras, Almut Beige, Karoline Wiesner
12 February 2010

Abstract:
Stochastic finite-state generators are compressed descriptions of infinite time series. Alternatively, compressed descriptions are given by quantum finite- state generators [K. Wiesner and J. P. Crutchfield, Physica D 237, 1173 (2008)]. These are based on repeated von Neumann measurements on a quantum dynamical system. Here we generalise the quantum finite-state generators by replacing the von Neumann pro jections by stochastic quantum operations. In this way we assure that any time series with a stochastic compressed description has a compressed quantum description. Moreover, we establish a link between our stochastic generators and the sequential readout of many-body states with translationally-invariant matrix product state representations. As an example, we consider the non-adaptive read-out of 1D cluster states. This is shown to be equivalent to a Hidden Quantum Model with two internal states, providing insight on the inherent complexity of the process. Finally, it is proven by example that the quantum description can have a higher degree of compression than the classical stochastic one.
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• Motional effects on the efficiency of excitation transfer
Ali Asadian, Markus Tiersch, Gian Giacomo Guerreschi, Jianming Cai, Sandu Popescu, Hans J. Briegel
03 February 2010

Abstract:
Energy transfer plays a vital role in many natural and technological processes. In this work, we study the effects of mechanical motion on the excitation transfer through a chain of interacting molecules with application to the biological scenario of energy transfer in $\alpha$-helices. Our investigation demonstrates that, for various types of mechanical oscillations, the transfer efficiency is significantly enhanced over that of comparable static configurations. This enhancement is a genuine quantum signature, and requires the collaborative interplay between the quantum-coherent evolution of the excitation and the mechanical motion of the molecules via their distance-dependent coupling; it has no analogue in the classical incoherent energy transfer. This effect may not only occur naturally, but it could be exploited in artificially designed systems to optimize transport processes. As an application, we discuss simple and hence robust control techniques.
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• Demonstration of unconditional one-way quantum computations for continuous variables
Ryuji Ukai, Noriaki Iwata, Yuji Shimokawa, Seiji C. Armstrong, Alberto Politi, Jun-ichi Yoshikawa, Peter van Loock, Akira Furusawa
28 January 2010

Abstract:
One-way quantum computation, realizable through measurements on a highly entangled state with no need for controlled unitary evolutions, is a very promising approach to fulfil the capabilities of quantum information processing. We demonstrate unconditional one-way quantum computation experiments using a linear cluster state of four entangled optical modes. A continuous quantum variable in phase space serves as the computational basis |x>. The key element of the continuous-variable scheme is that it does not rely on postselection or any probabilistic events; it works unconditionally, an advantage for scalability of quantum computation. We implement an important set of quantum operations in the optical phase space through one-way computation: Fourier rotations and squeezing. Though not sufficient, these linear transformations are necessary for universal quantum computation over continuous variables, and in our scheme, in principle, any such linear transformation can be unconditionally and deterministically applied to arbitrary single-mode quantum states. Hence our results provide a first demonstration of the fundamental components required for one-way quantum computation with continuous variables.
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• Semifluxon degeneracy choreography in Aharonov-Bohm billiards
M V Berry, S Popescu
14 January 2010

Abstract:
Every energy level of a charged quantum particle confined in a region threaded by a magnetic flux line with quantum flux one-half must be degenerate for some position of the semifluxon within the boundary B. This is illustrated by computations for which B is a circle and a conformal transformation of a circle without symmetry. As the shape of B is varied, two degeneracies between the same pair of levels can collide and annihilate. Degeneracy of three levels requires three shape parameters, or the positions of three semifluxons; degeneracy of N levels can be generated by int{N(N+1)/4} semifluxons. The force on the semifluxon is derived.
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• Nonadaptive quantum query algorithms for total functions
Ashley Montanaro
05 January 2010

Abstract:
We show that any bounded-error quantum query algorithm that computes some total boolean function depending on n variables, and whose queries to the input do not depend on the result of previous queries, must make Omega(n) queries to the input in total. Thus, in this restricted setting, quantum algorithms can achieve at most a constant factor speed-up over classical query algorithms.
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• An efficient test for product states
Aram Harrow, Ashley Montanaro
05 January 2010

Abstract:
We give a test that can distinguish efficiently between product states of n quantum systems and states which are far from product. If applied to a state psi whose maximum overlap with a product state is 1-epsilon, the test passes with probability 1-Theta(epsilon), regardless of n or the local dimensions of the individual systems. The test uses two copies of psi. We prove correctness of this test as a special case of a more general result regarding stability of maximum output purity of the depolarising channel.
One application of the test is to Quantum Merlin-Arthur games, where we show that a witness from two unentangled provers can simulate a witness from arbitrarily many unentangled provers, up to a constant loss of soundness. Our test can also be used to construct an efficient test for determining whether a unitary operator is a tensor product.
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• Quantum Reverse Shannon Theorem
Charles H. Bennett, Igor Devetak, Aram W. Harrow, Peter W. Shor, Andreas Winter
31 December 2009

Abstract:
We show how to use entanglement and noiseless quantum or classical communication to simulate discrete memoryless quantum channels with unit fidelity and efficiency in the limit of large block size. When the sender and receiver share enough standard ebits and are promised that the input to the channels is a memoryless (or i.i.d.) quantum source, our simulation uses an asymptotic rate of communication equal to the entanglement-assisted capacity of the channel. This communication rate also suffices for general (non-i.i.d.) sources if the ebits are replaced by a stronger entanglement resource, so-called entanglement-embezzling states, or if in addition to a supply of ebits, free backwards communication is allowed. Combined with previous coding theorems for entanglement-assisted classical communication over quantum channels, our results establish the ability of any channels to simulate any other, with an asymptotic efficiency given by the ratio of their entanglement-assisted capacities. Our result can be used to prove a strong converse to the coding theorem for entanglement-assisted classical communication.
We also give a regularized expression for the optimal communication-entanglement tradeoff for quantum channel simulation when a limited rate of ebits is available. We compare these quantum results with the analogous classical reverse Shannon theorem, and the analogous classical tradeoffs. We consider these tradeoffs both for ordinary simulations and for "feedback" simulations, where in the classical case the sender receives a copy of the channel's output and in the quantum case the sender receives the channel's environment.
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• Super-Duper-Activation of Quantum Zero-Error Capacities
Toby S. Cubitt, Graeme Smith
16 December 2009

Abstract:
The zero-error capacity of a channel is the rate at which it can send information perfectly, with zero probability of error, and has long been studied in classical information theory. We show that the zero-error capacity of quantum channels exhibits an extreme form of non-additivity, one which is not possible for classical channels, or even for the usual capacities of quantum channels. Building on the techniques of Ref. [1], combining probabilistic arguments with algebraic geometry, we prove that there exist channels E1 and E2 with no zero-error classical capacity whatsoever, C_0(E1) = C_0(E2) = 0, but whose joint zero-error quantum capacity is positive, Q_0(E1 x E2) >= 1. We term this striking effect "super-duper-activation", as it implies that both the classical and quantum zero-error capacities of these channels can be superactivated simultaneously, whilst being a strictly stronger property of capacities. Superactivation of the quantum zero-error capacity was not previously known.
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• Intra-molecular refrigeration in enzymes
Hans J. Briegel, Sandu Popescu
15 December 2009

Abstract:
We present a simple mechanism for intra-molecular refrigeration, where parts of a molecule are actively cooled below the environmental temperature. We discuss the potential role and applications of such a mechanism in biology, in particular in enzymatic reactions.
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• Improving zero-error classical communication with entanglement
Toby S. Cubitt, Debbie Leung, William Matthews, Andreas Winter
30 November 2009

Abstract:
Given one or more uses of a classical channel, only a certain number of messages can be transmitted with zero probability of error. The study of this number and its asymptotic behaviour constitutes the field of classical zero-error information theory, the quantum generalisation of which has started to develop recently. We show that, given a single use of certain classical channels, entangled states of a system shared by the sender and receiver can be used to increase the number of (classical) messages which can be sent with no chance of error. In particular, we show how to construct such a channel based on any proof of the Bell-Kochen-Specker theorem. This is a new example of the use of quantum effects to improve the performance of a classical task. We investigate the connection between this phenomenon and that of pseudo-telepathy'' games. The use of generalised non-signalling correlations to assist in this task is also considered. In this case, a particularly elegant theory results and, remarkably, it is sometimes possible to transmit information with zero-error using a channel with no unassisted zero-error capacity.
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• Measuring small longitudinal phase shifts: weak measurements or standard interferometry?
Nicolas Brunner, Christoph Simon
30 November 2009
Phys. Rev. Lett. 105, 010405 (2010)

Abstract:
Recently, weak measurements were used to measure small effects that are transverse to the propagation direction of a light beam. Here we address the question whether weak measurements are also useful for measuring small longitudinal phase shifts. We show that standard interferometry greatly outperforms weak measurements in a scenario involving a purely real weak value. However, we also present an interferometric scheme based on a purely imaginary weak value, combined with a frequency-domain analysis, which may have potential to outperform standard interferometry by several orders of magnitude.
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• Spontaneous Four-Wave Mixing of de Broglie Waves: Beyond Optics
Valentina Krachmalnicoff, Jean-Christophe Jaskula, Marie Bonneau, Guthrie B. Partridge, Denis Boiron, Christoph I Westbrook, Piotr Deuar, Pawel Zin, Marek Trippenbach, Karen Kheruntsyan
25 November 2009
Phys.Rev.Lett.104:150402,2010

Abstract:
We investigate the atom-optical analog of degenerate four-wave mixing of photons by colliding two Bose-Einstein condensates (BECs) of metastable helium and measuring the resulting momentum distribution of the scattered atoms with a time and space resolved detector. For the case of photons, phase matching conditions completely define the final state of the system, and in the case of two colliding BECs, simple analogy implies a spherical momentum distribution of scattered atoms. We find, however, that the final momenta of the scattered atoms instead lie on an ellipsoid whose radii are smaller than the initial collision momentum. Numerical and analytical calculations agree well with the measurements, and reveal the interplay between many-body effects, mean-field interaction, and the anisotropy of the source condensate.
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• Experimental feedback control of quantum systems using weak measurements
G. G. Gillett, R. B. Dalton, B. P. Lanyon, M. P. Almeida, M. Barbieri, G. J. Pryde, J. L. O'Brien, K. J. Resch, S. D. Bartlett, A. G. White
20 November 2009
Phys. Rev. Lett. 104, 080503 (2010)

Abstract:
A goal of the emerging field of quantum control is to develop methods for quantum technologies to function robustly in the presence of noise. Central issues are the fundamental limitations on the available information about quantum systems and the disturbance they suffer in the process of measurement. In the context of a simple quantum control scenario--the stabilization of non-orthogonal states of a qubit against dephasing--we experimentally explore the use of weak measurements in feedback control. We find that, despite the intrinsic difficultly of implementing them, weak measurements allow us to control the qubit better in practice than is even theoretically possible without them. Our work shows that these more general quantum measurements can play an important role for feedback control of quantum systems.
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• Manipulating multi-photon entanglement in waveguide quantum circuits
Jonathan C. F. Matthews, Alberto Politi, Andre Stefanov, Jeremy L. O'Brien
09 November 2009
Nature Photonics, 3, 346-350 (2009)

Abstract:
On-chip integrated photonic circuits are crucial to further progress towards quantum technologies and in the science of quantum optics. Here we report precise control of single photon states and multi-photon entanglement directly on-chip. We manipulate the state of path-encoded qubits using integrated optical phase control based on resistive elements, observing an interference contrast of 98.2+/-0.3%. We demonstrate integrated quantum metrology by observing interference fringes with 2- and 4-photon entangled states generated in a waveguide circuit, with respective interference contrasts of 97.2+/-0.4% and 92+/-4%, sufficient to beat the standard quantum limit. Finally, we demonstrate a reconfigurable circuit that continuously and accurately tunes the degree of quantum interference, yielding a maximum visibility of 98.2+/- 0.9%. These results open up adaptive and fully reconfigurable photonic quantum circuits not just for single photons, but for all quantum states of light.
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• Shor's quantum factoring algorithm on a photonic chip
Alberto Politi, Jonathan C. F. Matthews, Jeremy L. O'Brien
09 November 2009
Science, 325, 1221 (2009)

Abstract:
Shor's quantum factoring algorithm finds the prime factors of a large number exponentially faster than any other known method a task that lies at the heart of modern information security, particularly on the internet. This algorithm requires a quantum computer a device which harnesses the massive parallelism' afforded by quantum superposition and entanglement of quantum bits (or qubits). We report the demonstration of a compiled version of Shor's algorithm on an integrated waveguide silica-on-silicon chip that guides four single-photon qubits through the computation to factor 15.
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• The entanglement beam splitter: a quantum-dot spin in a double-sided optical microcavity
C.Y. Hu, W.J. Munro, J.L.O'Brien, J.G. Rarity
23 October 2009
Phys. Rev. B 80, 205326 (2009)

Abstract:
We propose an entanglement beam splitter (EBS) using a quantum-dot spin in a double-sided optical microcavity. In contrast to the conventional optical beam splitter, the EBS can directly split a photon-spin product state into two constituent entangled states via transmission and reflection with high fidelity and high efficiency (up to 100 percent). This device is based on giant optical circular birefringence induced by a single spin as a result of cavity quantum electrodynamics and the spin selection rule of trion transition (Pauli blocking). The EBS is robust and it is immune to the fine structure splitting in a realistic quantum dot. This quantum device can be used for deterministically creating photon-spin, photon-photon and spin-spin entanglement as well as a single-shot quantum non-demolition measurement of a single spin. Therefore, the EBS can find wide applications in quantum information science and technology.
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• Highly Entangled States With Almost No Secrecy
Matthias Christandl, Norbert Schuch, Andreas Winter
22 October 2009

Abstract:
In this paper we illuminate the relation between entanglement and secrecy by providing the first example of a quantum state that is highly entangled, but from which, nevertheless, almost no secrecy can be extracted. More precisely, we provide two bounds on the bipartite entanglement of the totally antisymmetric state in dimension d. First, we show that the amount of secrecy that can be extracted from the state is low, to be precise it is bounded by O(1/d). Second, we show that the state is highly entangled in the sense that we need a large amount of singlets to create the state: entanglement cost is larger than a constant, independent of d. In order to obtain our results we use representation theory, linear programming and the entanglement measure known as squashed entanglement.
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• Random tensor theory: extending random matrix theory to random product states
Andris Ambainis, Aram W. Harrow, Matthew B. Hastings
06 October 2009

Abstract:
We consider a problem in random matrix theory that is inspired by quantum information theory: determining the largest eigenvalue of a sum of p random product states in (C^d)^{otimes k}, where k and p/d^k are fixed while d grows. When k=1, the Marcenko-Pastur law determines (up to small corrections) not only the largest eigenvalue ((1+sqrt{p/d^k})^2) but the smallest eigenvalue (min(0,1-sqrt{p/d^k})^2) and the spectral density in between. We use the method of moments to show that for k>1 the largest eigenvalue is still approximately (1+sqrt{p/d^k})^2 and the spectral density approaches that of the Marcenko-Pastur law, generalizing the random matrix theory result to the random tensor case. Our bound on the largest eigenvalue has implications for a recently proposed quantum data hiding scheme due to Leung and Winter.
Since the matrices we consider have neither independent entries nor unitary invariance, we need to develop new techniques for their analysis. The main contribution of this paper is to give three different methods for analyzing mixtures of random product states: a diagrammatic approach based on Gaussian integrals, a combinatorial method that looks at the cycle decompositions of permutations and a recursive method that uses a variant of the Schwinger-Dyson equations.
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• Entropy and Information Causality in General Probabilistic Theories
Howard Barnum, Jonathan Barrett, Lisa Orloff Clark, Matthew Leifer, Robert Spekkens, Nicolas Stepanik, Alex Wilce, Robin Wilke
29 September 2009
to appear in New J. Phys.

Abstract:
We investigate the concept of entropy in probabilistic theories more general than quantum mechanics, with particular reference to the notion of information causality recently proposed by Pawlowski et. al. (arXiv:0905.2992). We consider two entropic quantities, which we term measurement and mixing entropy. In classical and quantum theory, they are equal, being given by the Shannon and von Neumann entropies respectively; in general, however, they are very different. In particular, while measurement entropy is easily seen to be concave, mixing entropy need not be. In fact, as we show, mixing entropy is not concave whenever the state space is a non-simplicial polytope. Thus, the condition that measurement and mixing entropies coincide is a strong constraint on possible theories. We call theories with this property monoentropic. Measurement entropy is subadditive, but not in general strongly subadditive. Equivalently, if we define the mutual information between two systems A and B by the usual formula I(A:B) = H(A) + H(B) - H(AB) where H denotes the measurement entropy and AB is a non-signaling composite of A and B, then it can happen that I(A:BC) < I(A:B). This is relevant to information causality in the sense of Pawlowski et al.: we show that any monoentropic non-signaling theory in which measurement entropy is strongly subadditive, and also satisfies a version of the Holevo bound, is informationally causal, and on the other hand we observe that Popescu-Rohrlich boxes, which violate information causality, also violate strong subadditivity. We also explore the interplay between measurement and mixing entropy and various natural conditions on theories that arise in quantum axiomatics.
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• On the communication complexity of XOR functions
Ashley Montanaro, Tobias Osborne
22 September 2009

Abstract:
An XOR function is a function of the form g(x,y) = f(x + y), for some boolean function f on n bits. We study the quantum and classical communication complexity of XOR functions. In the case of exact protocols, we show that, when f is monotone, g's quantum and classical complexities are quadratically related, and that when f is a linear threshold function, g's quantum complexity is Theta(n). More generally, we make a structural conjecture about the Fourier spectra of boolean functions which, if true, would imply that the quantum and classical deterministic communication complexities of all XOR functions are asymptotically equivalent. We give two randomised classical protocols for general XOR functions which are efficient for certain functions, and a third protocol for linear threshold functions with high margin. These protocols operate in the symmetric message passing model with shared randomness.
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• Closing the detection loophole in Bell experiments using qudits
Tamas Vertesi, Stefano Pironio, Nicolas Brunner
18 September 2009
Phys. Rev. Lett. 104, 060401 (2010)

Abstract:
We show that the detection efficiencies required for closing the detection loophole in Bell tests can be significantly lowered using quantum systems of dimension larger than two. We introduce a series of asymmetric Bell tests for which an efficiency arbitrarily close to 1/N can be tolerated using N-dimensional systems, and a symmetric Bell test for which the efficiency can be lowered down to 61.8% using four-dimensional systems. Experimental perspectives for our schemes look promising considering recent progress in atom-photon entanglement and in photon hyperentanglement.
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• Strong nonlocality: A trade-off between states and measurements
Anthony J. Short, Jonathan Barrett
15 September 2009
New Journal of Physics 12, 033034 (2010)

Abstract:
Measurements on entangled quantum states can produce outcomes that are nonlocally correlated. But according to Tsirelson's theorem, there is a quantitative limit on quantum nonlocality. It is interesting to explore what would happen if Tsirelson's bound were violated. To this end, we consider a model that allows arbitrary nonlocal correlations, colloquially referred to as "box world". We show that while box world allows more highly entangled states than quantum theory, measurements in box world are rather limited. As a consequence there is no entanglement swapping, teleportation or dense coding.
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• Entanglement spread and clean resource inequalities
Aram W. Harrow
10 September 2009

Abstract:
This article will examine states that superpose different amounts of entanglement and protocols that run in superposition but generate or consume different amounts of entanglement. In both cases we find a uniquely quantum difficulty: entanglement cannot be conditionally discarded without either using communication or causing decoherence.
I will first describe the problem of entanglement spread in states and operations, as well as some methods of dealing with it. Then I'll describe three applications to problems that at first glance appear to be quite different: first, a reinterpretation of the old observation that creating n partially entangled states from singlets requires theta(sqrt(n)) communication, but cannot itself be used to communicate; second, a new lower bound technique for communication complexity; third, an explanation of how to extend the quantum reverse Shannon theorem from tensor power sources to general sources.
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• Adaptive versus non-adaptive strategies for quantum channel discrimination
Aram W. Harrow, Avinatan Hassidim, Debbie W. Leung, John Watrous
03 September 2009
Phys. Rev. A 81, 032339 (2010)

Abstract:
We provide a simple example that illustrates the advantage of adaptive over non-adaptive strategies for quantum channel discrimination. In particular, we give a pair of entanglement-breaking channels that can be perfectly discriminated by means of an adaptive strategy that requires just two channel evaluations, but for which no non-adaptive strategy can give a perfect discrimination using any finite number of channel evaluations.
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• Quantum search with advice
Ashley Montanaro
24 August 2009
In Proc. TQC 2010

Abstract:
We consider the problem of search of an unstructured list for a marked element, when one is given advice as to where this element might be located, in the form of a probability distribution. The goal is to minimise the expected number of queries to the list made to find the marked element, with respect to this distribution. We present a quantum algorithm which solves this problem using an optimal number of queries, up to a constant factor. For some distributions on the input, such as certain power law distributions, the algorithm can achieve exponential speed-ups over the best possible classical algorithm. We also give an efficient quantum algorithm for a variant of this task where the distribution is not known in advance, but must be queried at an additional cost. The algorithms are based on the use of Grover's quantum search algorithm and amplitude amplification as subroutines.
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• Deciding whether a Quantum Channel is Markovian is NP-hard
Toby S. Cubitt, Jens Eisert, Michael M. Wolf
18 August 2009

Abstract:
We show that deciding whether a given quantum channel can be generated by a Markovian master equation is an NP-hard problem. As a consequence, this result suggests that extracting the underlying physics governing the evolution of a quantum system, as described by its dynamical equations, may be a hard task regardless of how much data is gathered via measurements of the system. On the other hand, if the system dimension is fixed, the problem of deciding whether a process is consistent with being Markovian can be efficiently decided, and we give an explicit algorithm in this case. This work also leads to a complexity-theoretic solution to a long-standing open problem in probability theory.
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• How small can thermal machines be? Towards the smallest possible refrigerator
Noah Linden, Sandu Popescu, Paul Skrzypczyk
17 August 2009

Abstract:
We investigate the fundamental dimensional limits to thermodynamic machines. In particular we show that it is possible to construct self-contained refrigerators (i.e. not requiring external sources of work) consisting of only a small number of qubits and/or qutrits. We present three different models, one consisting of two qubits, one consisting of a qubit and a qutrit with nearest-neighbour interactions, and one consisting of a single qutrit. We then investigate fundamental limits to their performance; in particular we show that it is possible to cool towards absolute zero.
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• Matchgate and space-bounded quantum computations are equivalent
Richard Jozsa, Barbara Kraus, Akimasa Miyake, John Watrous
12 August 2009
Proc. R. Soc. A 466, 809-830 (2010)

Abstract:
Matchgates are an especially multiflorous class of two-qubit nearest neighbour quantum gates, defined by a set of algebraic constraints. They occur for example in the theory of perfect matchings of graphs, non-interacting fermions, and one-dimensional spin chains. We show that the computational power of circuits of matchgates is equivalent to that of space-bounded quantum computation with unitary gates, with space restricted to being logarithmic in the width of the matchgate circuit. In particular, for the conventional setting of polynomial-sized (logarithmic-space generated) families of matchgate circuits, known to be classically simulatable, we characterise their power as coinciding with polynomial-time and logarithmic-space bounded universal unitary quantum computation.
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• Closure of theories with limited non-locality
Jonathan Allcock, Nicolas Brunner, Noah Linden, Sandu Popescu, Paul Skrzypczyk, Tamas Vertesi
12 August 2009
Phys. Rev. A 80, 062107 (2009)

Abstract:
An intensive research effort has recently been devoted to understanding the properties of general non-signaling theories, which can contain more non-locality than quantum mechanics. Here we argue that in order to form self-consistent theories, sets of non-signaling correlations with limited non-locality must be closed under a natural class of operations called wirings. After introducing useful concepts and tools to address the issue of closure, we present several case studies. Furthermore we discuss the implications of our findings in the broader context of this line of research, in particular concerning the origin of the boundary between quantum and post-quantum correlations, and towards finding constraints on physical theories beyond quantum mechanics.
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• Tripartite to Bipartite Entanglement Transformations and Polynomial Identity Testing
Eric Chitambar, Runyao Duan, Yaoyun Shi
05 August 2009

Abstract:
We consider the problem of deciding if a given three-party entangled pure state can be converted, with a non-zero success probability, into a given two-party pure state through local quantum operations and classical communication. We show that this question is equivalent to the well-known computational problem of deciding if a multivariate polynomial is identically zero. Efficient randomized algorithms developed to study the latter can thus be applied to the question of tripartite to bipartite entanglement transformations.
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• Quantum algorithms for testing properties of distributions
Sergey Bravyi, Aram W. Harrow, Avinatan Hassidim
24 July 2009
to appear STACS 2010

Abstract:
Suppose one has access to oracles generating samples from two unknown probability distributions P and Q on some N-element set. How many samples does one need to test whether the two distributions are close or far from each other in the L_1-norm ? This and related questions have been extensively studied during the last years in the field of property testing. In the present paper we study quantum algorithms for testing properties of distributions. It is shown that the L_1-distance between P and Q can be estimated with a constant precision using approximately N^{1/2} queries in the quantum settings, whereas classical computers need \Omega(N) queries. We also describe quantum algorithms for testing Uniformity and Orthogonality with query complexity O(N^{1/3}). The classical query complexity of these problems is known to be \Omega(N^{1/2}).
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• All Mutually Unbiased Bases in Dimensions Two to Five
Stephen Brierley, Stefan Weigert, Ingemar Bengtsson
23 July 2009
Quantum Info. & Comp. Vol 10, 0803 (2010)

Abstract:
All complex Hadamard matrices in dimensions two to five are known. We use this fact to derive all inequivalent sets of mutually unbiased (MU) bases in low dimensions. We find a three-parameter family of triples of MU bases in dimension four and two inequivalent classes of MU triples in dimension five. We confirm that the complete sets of (d+1) MU bases are unique (up to equivalence) in dimensions below six, using only elementary arguments for d less than five.
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• Entropic uncertainty relations - A survey
Stephanie Wehner, Andreas Winter
22 July 2009

Abstract:
Uncertainty relations play a central role in quantum mechanics. Entropic uncertainty relations in particular have gained significant importance within quantum information, providing the foundation for the security of many quantum cryptographic protocols. Yet, rather little is known about entropic uncertainty relations with more than two measurement settings. In this note we review known results and open questions.
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• Learning and Testing Algorithms for the Clifford Group
Richard Low
17 July 2009
Phys. Rev. A Vol 80, 052314 (2009)

Abstract:
Given oracle access to an unknown unitary C from the Clifford group and its conjugate, we give an exact algorithm for identifying C with O(n) queries, which we prove is optimal. We then extend this to all levels of the Gottesman-Chuang hierarchy (also known as the C_k hierarchy). Further, for unitaries not in the hierarchy itself but known to be close to an element of the hierarchy, we give a method of finding this close element. We also present a Clifford testing algorithm that decides whether a given black-box unitary is close to a Clifford or far from every Clifford.
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• Violation of the Leggett-Garg inequality with weak measurements of photons
M. E. Goggin, M. P. Almeida, M. Barbieri, B. P. Lanyon, J. L. O'Brien, A. G. White, G. J. Pryde
13 July 2009

Abstract:
By weakly measuring the polarization of a photon between two strong polarization measurements, we experimentally investigate the correlation between the appearance of anomalous values in quantum weak measurements, and the violation of realism and non-intrusiveness of measurements. A quantitative formulation of the latter concept is expressed in terms of a Leggett-Garg inequality for the outcomes of subsequent measurements of an individual quantum system. We experimentally violate the Leggett-Garg inequality for several measurement strengths. Furthermore, we experimentally demonstrate that there is a one-to-one correlation between achieving strange weak values and violating the Leggett-Garg inequality.
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• A Novel Phase Shift Acquired due to Virtual Forces
T. Kaufherr, Y. Aharonov, S. Nussinov, S. Popescu, J. Tollaksen
13 July 2009

Abstract:
In the following we discuss a simple one dimensional scattering problem involving a strong short-range interaction between a heavy and a light particle. It allows to introduce concepts of "potential", virtual forces and "private" potentials, which explain the phase acquired by the heavy particle during the entanglement /scattering event.
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• On the speed of fluctuations around thermodynamic equilibrium
Noah Linden, Sandu Popescu, Anthony J Short, Andreas Winter
08 July 2009

Abstract:
We study the speed of fluctuation of a quantum system around its thermodynamic equilibrium state, and show that the speed will be extremely small for almost all times in typical thermodynamic cases. The setting considered here is that of a quantum system couples to a bath, both jointly described as a closed system. This setting, is the same as the one considered in [N. Linden et al., Phys. Rev. E 79:061103 (2009)] and the thermodynamic equilibrium state'' refers to a situation that includes the usual thermodynamic equilibrium case, as well as far more general situations.
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• Dynamics of Entanglement and Attractor' states in The Tavis-Cummings Model
C. E. A. Jarvis, D. A. Rodrigues, B. L. Györffy, T. P. Spiller, A. J. Short, J. F. Annett
23 June 2009

Abstract:
We study the time evolution of $N_q$ two-level atoms (or qubits) interacting with a single mode of the quantised radiation field. In the case of two qubits, we show that for a set of initial conditions the reduced density matrix of the atomic system approaches that of a pure state at $\sfrac{t_r}{4}$, halfway between that start of the collapse and the first mini revival peak, where $t_r$ is the time of the main revival. The pure state approached is the same for a set of initial conditions and is thus termed an attractor state'. The set itself is termed the basin of attraction and the features are at the center of our attention. Extending to more qubits, we find that attractors are a generic feature of the multi qubit Jaynes Cummings model (JCM) and we therefore generalise the discovery by Gea-Banacloche for the one qubit case. We give the basin of attraction' for $N_q$ qubits and discuss the implications of the attractor' state in terms of the dynamics of $N_q$-body entanglement. We observe both collapse and revival and sudden birth/death of entanglement depending on the initial conditions.
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• Inhomogeneous Quantum Walks
Noah Linden, James Sharam
22 June 2009
Phys. Rev. A 80, 052327 (2009)

Abstract:
We study a natural construction of a general class of inhomogeneous quantum walks (namely walks whose transition probabilities depend on position). Within the class we analyze walks that are periodic in position and show that, depending on the period, such walks can be bounded or unbounded in time; in the latter case we analyze the asymptotic speed. We compare the construction to others in the existing literature. As an example we give a quantum version of a non-irreducible classical walk: the Polya Urn.
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• Recovering part of the quantum boundary from information causality
Jonathan Allcock, Nicolas Brunner, Marcin Pawlowski, Valerio Scarani
19 June 2009
Phys. Rev. A 80, 040103(R) (2009)

Abstract:
Recently, the principle of information causality has appeared as a good candidate for an information-theoretic principle that would single out quantum correlations among more general non-signalling models. Here we present results going in this direction; namely we show that part of the boundary of quantum correlations actually emerges from information causality.
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• Superactivation of the Asymptotic Zero-Error Classical Capacity of a Quantum Channel
Toby S. Cubitt, Jianxin Chen, Aram W. Harrow
16 June 2009

Abstract:
The zero-error classical capacity of a quantum channel is the asymptotic rate at which it can be used to send classical bits perfectly, so that they can be decoded with zero probability of error. We show that there exist pairs of quantum channels, neither of which individually have any zero-error capacity whatsoever (even if arbitrarily many uses of the channels are available), but such that access to even a single copy of both channels allows classical information to be sent perfectly reliably. In other words, we prove that the zero-error classical capacity can be superactivated. This result is the first example of superactivation of a classical capacity of a quantum channel.
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• Unconditional security from noisy quantum storage
Robert Koenig, Stephanie Wehner, Juerg Wullschleger
08 June 2009

Abstract:
We consider the implementation of two-party cryptographic primitives based on the sole assumption that no large-scale reliable quantum storage is available to the cheating party. We construct novel protocols for oblivious transfer and bit commitment, and prove that realistic noise levels provide security even against the most general attack. Such unconditional results were previously only known in the so-called bounded-storage model which is a special case of our setting. Our protocols can be implemented with present-day hardware used for quantum key distribution. In particular, no quantum storage is required for the honest parties.
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• A new physical principle: Information Causality
M. Pawlowski, T. Paterek, D. Kaszlikowski, V. Scarani, A. Winter, M. Zukowski
15 May 2009
Nature 461, 1101 (2009)

Abstract:
Quantum physics exhibits many remarkable features. For example, it gives probabilistic predictions (non-determinism), does not allow copying of unknown states (no-cloning), its correlations are stronger than any classical correlations but information cannot be transmitted faster than light (no-signaling). However, all the mentioned features do not single out quantum physics. A broad class of theories exist which share all of them with quantum mechanics and allow even stronger than quantum correlations. Here, we introduce the principle of Information Causality, stating that communication of m classical bits causes information gain of at most m bits. We show that this principle is respected both in classical and quantum physics, and that all stronger than quantum correlations violate it. We suggest that Information Causality, being a generalization of no-signaling, is one of the foundational properties of Nature.
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• An Entanglement Filter
Ryo Okamoto, Jeremy L. O'Brien, Holger F. Hofmann, Tomohisa Nagata, Keiji Sasaki, Shigeki Takeuchi
04 May 2009
Science 323, 483 (2009)

Abstract:
The ability to filter quantum states is a key capability in quantum information science and technology, in which one-qubit filters, or polarizers, have found wide application. Filtering on the basis of entanglement requires extension to multi-qubit filters with qubit-qubit interactions. We demonstrated an optical entanglement filter that passes a pair of photons if they have the desired correlations of their polarization. Such devices have many important applications to quantum technologies.
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• Symmetric functions of qubits in an unknown basis
Ashley Montanaro
01 April 2009
Phys. Rev. A 79, 062316, 2009

Abstract:
Consider an n qubit computational basis state corresponding to a bit string x, which has had an unknown local unitary applied to each qubit, and whose qubits have been reordered by an unknown permutation. We show that, given such a state with Hamming weight |x| at most n/2, it is possible to reconstruct |x| with success probability 1 - |x|/(n-|x|+1), and thus to compute any symmetric function of x. We give explicit algorithms for computing whether or not |x| is at least t for some t, and for computing the parity of x, and show that these are essentially optimal. These results can be seen as generalisations of the swap test for comparing quantum states.
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• Large Deviation Bounds for k-designs
Richard Low
31 March 2009
Proc. R. Soc. A, 465(2111):3289-3308, 2009

Abstract:
We present a technique for derandomising large deviation bounds of functions on the unitary group. We replace the Haar distribution with a pseudo-random distribution, a k-design. k-designs have the first k moments equal to those of the Haar distribution. The advantage of this is that (approximate) k-designs can be implemented efficiently, whereas Haar random unitaries cannot. We find large deviation bounds for unitaries chosen from a k-design and then illustrate this general technique with three applications. We first show that the von Neumann entropy of a pseudo-random state is almost maximal. Then we show that, if the dynamics of the universe produces a k-design, then suitably sized subsystems will be in the canonical state, as predicted by statistical mechanics. Finally we show that pseudo-random states are useless for measurement based quantum computation.
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• Device-independent quantum key distribution secure against collective attacks
Stefano Pironio, Antonio Acin, Nicolas Brunner, Nicolas Gisin, Serge Massar, Valerio Scarani
27 March 2009
New J. Phys. 11, 045021 (2009)

Abstract:
Device-independent quantum key distribution (DIQKD) represents a relaxation of the security assumptions made in usual quantum key distribution (QKD). As in usual QKD, the security of DIQKD follows from the laws of quantum physics, but contrary to usual QKD, it does not rely on any assumptions about the internal working of the quantum devices used in the protocol. We present here in detail the security proof for a DIQKD protocol introduced in [Phys. Rev. Lett. 98, 230501 (2008)]. This proof exploits the full structure of quantum theory (as opposed to other proofs that exploit the no-signalling principle only), but only holds again collective attacks, where the eavesdropper is assumed to act on the quantum systems of the honest parties independently and identically at each round of the protocol (although she can act coherently on her systems at any time). The security of any DIQKD protocol necessarily relies on the violation of a Bell inequality. We discuss the issue of loopholes in Bell experiments in this context.
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• Nonadditivity of the Private Classical Capacity of a Quantum Channel
Ke Li, Andreas Winter, XuBo Zou, GuangCan Guo
26 March 2009

Abstract:
We construct a family of quantum channels for which we can bound the classical capacity by their Holevo capacity plus an arbitrarily small correction. On the other hand, their quantum capacity when combined with a zero private (and zero quantum) capacity erasure channel, becomes larger than the previous Holevo capacity.
As a consequence, we can conclude that the classical private capacity is nonadditive. In fact, in our construction even the quantum capacity of the tensor product of two channels can be greater than the sum of their individual classical private capacities.
We show that this violation occurs quite generically: every channel can be embedded into our construction, and a violation occurs whenever the given channel has larger entanglement assisted quantum capacity than (unassisted) classical capacity.
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• Arbitrarily little knowledge can give a quantum advantage for nonlocal tasks
Jonathan Allcock, Harry Buhrman, Noah Linden
04 March 2009
Phys. Rev. A 80, 032105 (2009)

Abstract:
It has previously been shown that quantum nonlocality offers no benefit over classical correlations for performing a distributed task known as nonlocal computation. This is where separated parties must compute the value of a function without individually learning anything about the inputs. We show that giving the parties some knowledge of the inputs, however small, is sufficient to unlock the power of quantum mechanics to out-perform classical mechanics. This role of information held locally gives new insight into the general question of when quantum nonlocality gives an advantage over classical physics. Our results also reveal a novel feature of the nonlocality embodied in the celebrated task of Clauser, Horne, Shimony and Holt.
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• Laser written waveguide photonic quantum circuits
G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, J. L. O'Brien
26 February 2009
Optics Express, Vol. 17, No. 15, pp. 12546-12554, 20 July 2009.

Abstract:
We report photonic quantum circuits created using an ultrafast laser processing technique that is rapid, requires no lithographic mask and can be used to create three-dimensional networks of waveguide devices. We have characterized directional couplers--the key functional elements of photonic quantum circuits--and found that they outperform previous lithographically produced waveguide devices. We further demonstrate high-performance interferometers and an important multi-photon quantum interference phenomenon for the first time in integrated optics.This direct-write approach will enable the rapid development of sophisticated quantum optical circuits and their scaling into three-dimensions.
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• Quantum experiments with human eyes as detectors based on cloning via stimulated emission
Pavel Sekatski, Nicolas Brunner, Cyril Branciard, Nicolas Gisin, Christoph Simon
18 February 2009
Phys. Rev. Lett. 103, 113601 (2009)

Abstract:
We show theoretically that the multi-photon states obtained by cloning single-photon qubits via stimulated emission can be distinguished with the naked human eye with high efficiency and fidelity. Focusing on the "micro-macro" situation realized in a recent experiment [F. De Martini, F. Sciarrino, and C. Vitelli, Phys. Rev. Lett. 100, 253601 (2008)], where one photon from an original entangled pair is detected directly, whereas the other one is greatly amplified, we show that performing a Bell experiment with human-eye detectors for the amplified photon appears realistic, even when losses are taken into account. The great robustness of these results under photon loss leads to an apparent paradox, which we resolve by noting that the Bell violation proves the existence of entanglement before the amplification process. However, we also prove that there is genuine micro-macro entanglement even for high loss.
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• Quantum mutual independence
Michal Horodecki, Jonathan Oppenheim, Andreas Winter
06 February 2009

Abstract:
We introduce the concept of mutual independence -- correlations shared between distant parties which are independent of the environment. This notion is more general than the standard idea of a secret key -- it is a fully quantum and more general form of privacy. The states which possess mutual independence also generalize the so called private states -- those that possess private key. We then show that the problem of distributed compression of quantum information at distant sources can be solved in terms of mutual independence, if free entanglement between the senders and the receiver is available. Namely, we obtain a formula for the sum of rates of qubits needed to transmit a distributed state between Alice and Bob to a decoder Charlie. We also show that mutual independence is bounded from above by the relative entropy modulo a conjecture, saying that if after removal of a single qubit the state becomes product, its initial entanglement is bounded by 1. We suspect that mutual independence is a highly singular quantity, i.e. that it is positive only on a set of measure zero; furthermore, we believe that its presence is seen on the single copy level. This appears to be born out in the classical case.
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• Robust photon-spin entangling gate using a quantum-dot spin in a microcavity
C.Y.Hu, W.J.Munro, J.L.O'Brien, J.G. Rarity
27 January 2009

Abstract:
Semiconductor quantum dots (known as artificial atoms) hold great promise for solid-state quantum networks and quantum computers. To realize a quantum network, it is crucial to achieve light-matter entanglement and coherent quantum-state transfer between light and matter. Here we present a robust photon-spin entangling gate with high fidelity and high efficiency (up to 50 percent) using a charged quantum dot in a double-sided microcavity. This gate is based on giant circular birefringence induced by a single electron spin, and functions as an optical circular polariser which allows only one circularly-polarized component of light to be transmitted depending on the electron spin states. We show this gate can be used for single-shot quantum non-demolition measurement of a single electron spin, and can work as an entanglement filter to make a photon-spin entangler, spin entangler and photon entangler as well as a photon-spin quantum interface. This work allows us to make all building blocks for solid-state quantum networks with single photons and quantum-dot spins.
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• Non-locality distillation and post-quantum theories with trivial communication complexity
Nicolas Brunner, Paul Skrzypczyk
27 January 2009
Phys. Rev. Lett. 102, 160403 (2009)

Abstract:
We first present a protocol for deterministically distilling non-locality, which is optimal under a general assumption. In particular our protocol works efficiently for a specific class of post-quantum non-local boxes, which we term correlated non-local boxes. In the asymptotic limit, all correlated non-local boxes are distilled to the maximally non-local box, the Popescu-Rohrlich box. Then, taking advantage of a recent result of Brassard et al. [Phys. Rev. Lett. 96, 250401 (2006)] we show that all correlated non-local boxes make communication complexity trivial, and therefore appear very unlikely to exist in nature. Astonishingly, some of these non-local boxes are arbitrarily close to the set of classical correlations. This result therefore gives new insight to the problem of why quantum non-locality is limited.
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• Constructing Mutually Unbiased Bases in Dimension Six
Stephen Brierley, Stefan Weigert
26 January 2009
Phys. Rev. A 79, 052316 (2009)

Abstract:
The density matrix of a qudit may be reconstructed with optimal efficiency if the expectation values of a specific set of observables are known. In dimension six, the required observables only exist if it is possible to identify six mutually unbiased complex 6x6 Hadamard matrices. Prescribing a first Hadamard matrix, we construct all others mutually unbiased to it, using algebraic computations performed by a computer program. We repeat this calculation many times, sampling all known complex Hadamard matrices, and we never find more than two that are mutually unbiased. This result adds considerable support to the conjecture that no seven mutually unbiased bases exist in dimension six.
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• Nonclassical 2-photon interference with separate intrinsically narrowband fibre sources
Matthaeus Halder, Jeremie Fulconis, Ben Cemlyn, Alex Clark, Chunle Xiong, William J. Wadsworth, John G. Rarity
20 January 2009
Optics Express, Vol. 17, Issue 6, pp. 4670-4676 (2009)

Abstract:
In this paper, we demonstrate a source of photon pairs based on four-wave-mixing in photonic crystal fibres. Careful engineering of the phase matching conditions in the fibres enables us to create photon pairs at 597nm and 860nm in an intrinsically time-bandwidth limited state which, furthermore, shows no spectral correlations. This allows for heralding one photon in a pure state and hence renders narrow band filtering obsolete. The source is narrow band, bright and achieves an overall detection efficiency of up to 21% per photon. For the first time, a Hong-Ou-Mandel interference with unfiltered photons from separate fibre sources is presented.
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• Embedding classical into quantum computation
Richard Jozsa
25 December 2008
LNCS 5393 Beth Festschrift, J Calmet, W. Geiselmann, J. Mueller-Quade (Eds.), pp43-49 (2008)

Abstract:
We describe a simple formalism for generating classes of quantum circuits that are classically efficiently simulatable and show that the efficient simulation of Clifford circuits (Gottesman-Knill theorem) and of matchgate circuits (Valiant's theorem) appear as two special cases. Viewing these simulatable classes as subsets of the space of all quantum computations, we may consider minimal extensions that suffice to regain full quantum computational power, which provides an approach to exploring the efficacy of quantum over classical computation.
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• Collapse and Revival of Schrödinger Cat' States
C. E. A. Jarvis, D. A. Rodrigues, B. L. Györffy, T. P. Spiller, A. J. Short, J. F. Annett
22 December 2008

Abstract:
We study the dynamics of the Jaynes-Cummings Model for an array of $N_q$ two level systems (or qubits) interacting with a quantized single mode electromagnetic cavity (or quantum bus). For an initial cavity coherent state $| \alpha >$ and the qubit system in a specified basin of attraction' in its Hilbert space, we demonstrate the oscillation of a superposition of two macroscopic quantum states between the qubit system and the field mode. From the perspective of either the qubit or the field system, there is collapse and revival of a Schr\"odinger Cat' state.
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• Are random pure states useful for quantum computation?
Michael J. Bremner, Caterina Mora, Andreas Winter
17 December 2008

Abstract:
We show the following: a randomly chosen pure state as a resource for measurement-based quantum computation, is - with overwhelming probability - of no greater help to a polynomially bounded classical control computer, than a string of random bits. Thus, unlike the familiar "cluster states", the computing power of a classical control device is not increased from P to BQP, but only to BPP. The same holds if the task is to sample from a distribution rather than to perform a bounded-error computation. Furthermore, we show that our results can be extended to states with significantly less entanglement than random states.
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• Quantum mechanical evolution towards thermal equilibrium
Noah Linden, Sandu Popescu, Anthony J. Short, Andreas Winter
15 December 2008

Abstract:
The circumstances under which a system reaches thermal equilibrium, and how to derive this from basic dynamical laws, has been a major question from the very beginning of thermodynamics and statistical mechanics. Despite considerable progress, it remains an open problem. Motivated by this issue, we address the more general question of equilibration. We prove, with virtually full generality, that reaching equilibrium is a universal property of quantum systems: Almost any subsystem in interaction with a large enough bath will reach an equilibrium state and remain close to it for almost all times. We also prove several general results about other aspects of thermalisation besides equilibration, for example, that the equilibrium state does not depend on the detailed micro-state of the bath.
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• Couplers for Non-Locality Swapping
Paul Skrzypczyk, Nicolas Brunner
04 December 2008
New Journal of Physics 11, 073014, (2009)

Abstract:
Studying generalized non-local theories brings insight to the foundations of quantum mechanics. Here we focus on non-locality swapping, the analogue of quantum entanglement swapping. In order to implement such a protocol, one needs a coupler that performs the equivalent of quantum joint measurements on generalized box-like' states. Establishing a connection to Bell inequalities, we define consistent couplers for theories containing an arbitrary amount of non-locality, which leads us to introduce the concepts of perfect and minimal couplers. Remarkably, Tsirelson's bound for quantum non-locality naturally appears in our study.
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• Bit Commitment from Non-Local Correlations
Severin Winkler, Stefan Wolf, Juerg Wullschleger
24 November 2008

Abstract:
Central cryptographic functionalities such as encryption, authentication, or secure two-party computation cannot be realized in an information-theoretically secure way from scratch. This serves as a motivation to study what (possibly weak) primitives they can be based on. We consider as such starting points general two-party input-output systems that do not allow for message transmission, and show that they can be used for realizing unconditionally secure bit commitment as soon as they are non-trivial, i.e., cannot be realized from distributed randomness only. In particular, our result implies that any two-qubit state without hidden-variable model has an input-output behavior allowing for unconditional bit commitment.
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• Quantum algorithm for solving linear systems of equations
Aram W. Harrow, Avinatan Hassidim, Seth Lloyd
20 November 2008
Phys. Rev. Lett. vol. 15, no. 103, pp. 150502 (2009)

Abstract:
Solving linear systems of equations is a common problem that arises both on its own and as a subroutine in more complex problems: given a matrix A and and an vector b, find a vector x such that Ax=b.
Often, one does not need to know the solution x itself, but rather an approximation of the expectation value of some operator associated with x, e.g., x^\dag M x for some matrix M. In this case, when A is sparse and well-conditioned, with largest dimension n, the best classical algorithms can find x and estimate x^\dag M x in O(n) time. Here, we exhibit a quantum algorithm for solving linear sets of equations that runs in O(log n) time, an exponential improvement over the best classical algorithm.
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• Emergence of Quantum Mechanics from Non-Locality Swapping
Paul Skrzypczyk, Nicolas Brunner, Sandu Popescu
19 November 2008
Phys. Rev. Lett. 102, 110402 (2009)

Abstract:
Recently the study of general non-signalling theories has brought a lot of insight to quantum mechanics. By investigating these generalized models, the hope is to find out what is essential in quantum mechanics; what makes it so special. Answering this question will definitely provide a deeper understanding of the foundations of quantum mechanics, and may enable further developments in quantum information science. In the present paper, we revisit the paradigmatic model of non-signalling boxes and introduce the concept of a genuine box. This will allow us to present the first generalized non-signalling model featuring quantum-like dynamics. In particular, we present the coupler, a device enabling non-locality swapping, the analogue of quantum entanglement swapping, as well as teleportation. Remarkably, a clear boundary between quantum and post-quantum correlations naturally emerges in our study.
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• Efficient Quantum Tensor Product Expanders and k-designs
Aram W. Harrow, Richard A. Low
18 November 2008
Proceedings of RANDOM 2009, LNCS, 5687:548-561, 2009

Abstract:
We give an efficient construction of constant-degree, constant-gap quantum k-tensor product expanders. The key ingredients are an efficient classical tensor product expander and the quantum Fourier transform. Our construction works whenever k=O(n/log n), where n is the number of qubits. An immediate corollary of this result is an efficient construction of approximate unitary k-designs on n qubits for any k=O(n/log n).
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• Distinguishability of quantum states under restricted families of measurements with an application to quantum data hiding
William Matthews, Stephanie Wehner, Andreas Winter
15 October 2008

Abstract:
Every sufficiently rich set of measurements on a fixed quantum system defines a statistical norm on the states of that system via the optimal bias that can be achieved in distinguishing the states using measurements from that set (assuming equal priors). The Holevo-Helstrom theorem says that for the set of all measurements this norm is the trace norm. For finite dimension any norm is lower and upper bounded by constant (though dimension dependent) multiples of the trace norm, so we set ourselves the task of computing or bounding the best possible "constants of domination" for the norms corresponding to various restricted sets of measurements, thereby determining the worst case and best case performance of these sets relative to the set of all measurements.
We look at the case where the allowed set consists of a single measurement, namely the uniformly random continuous POVM and its approximations by 2-designs and 4-designs respectively. Here we find asymptotically tight bounds for the constants of domination.
Furthermore, we analyse the multipartite setting with any LOCC measurement allowed. In the case of two parties, we show that the lower domination constant is the same as that of a tensor product of local uniformly random POVMs (up to a constant). This answers in the affirmative an open question about the (near-)optimality of bipartite data hiding: The bias that can be achieved by LOCC in discriminating two orthogonal states of a d x d bipartite system is Omega(1/d), which is known to be tight. Finally, we use our analysis to derive certainty relations (in the sense of Sanchez-Ruiz) for any such measurements and to lower bound the locally accessible information for bipartite systems.
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• Quantum boolean functions
Ashley Montanaro, Tobias J. Osborne
15 October 2008
Chicago Journal of Theoretical Computer Science, to appear

Abstract:
In this paper we introduce the study of quantum boolean functions, which are unitary operators f whose square is the identity: f^2 = I. We describe several generalisations of well-known results in the theory of boolean functions, including quantum property testing; a quantum version of the Goldreich-Levin algorithm for finding the large Fourier coefficients of boolean functions; and two quantum versions of a theorem of Friedgut, Kalai and Naor on the Fourier spectra of boolean functions. In order to obtain one of these generalisations, we prove a quantum extension of the hypercontractive inequality of Bonami, Gross and Beckner.
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• Testing a Bell inequality in multi-pair scenarios
Jean-Daniel Bancal, Cyril Branciard, Nicolas Brunner, Nicolas Gisin, Sandu Popescu, Christoph Simon
07 October 2008
Phys. Rev. A 78, 062110 (2008)

Abstract:
To date, most efforts to demonstrate quantum nonlocality have concentrated on systems of two (or very few) particles. It is however difficult in many experiments to address individual particles, making it hard to highlight the presence of nonlocality. We show how a natural setup with no access to individual particles allows one to violate the CHSH inequality with many pairs, including in our analysis effects of noise and losses. We discuss the case of distinguishable and indistinguishable particles. Finally, a comparison of these two situations provides new insight into the complex relation between entanglement and nonlocality.
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• Dynamic entanglement in oscillating molecules
Jianming Cai, Sandu Popescu, Hans J. Briegel
30 September 2008

Abstract:
We demonstrate that entanglement can persistently recur in an oscillating two-spin molecule that is coupled to a hot and noisy environment, in which no static entanglement can survive. The system represents a non-equilibrium quantum system which, driven through the oscillatory motion, is prevented from reaching its (separable) thermal equilibrium state. Environmental noise, together with the driven motion, plays a constructive role by periodically resetting the system, even though it will destroy entanglement as usual. As a building block, the present simple mechanism supports the perspective that entanglement can exist also in systems which are exposed to a hot environment and to high levels of de-coherence, which we expect e.g. for biological systems. Our results furthermore suggest that entanglement plays a role in the heat exchange between molecular machines and environment. Experimental simulation of our model with trapped ions is within reach of the current state-of-the-art quantum technologies.
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• Oblivious Transfer from Weak Noisy Channels
Jürg Wullschleger
30 September 2008

Abstract:
Various results show that oblivious transfer can be implemented using the assumption of noisy channels. Unfortunately, this assumption is not as weak as one might think, because in a cryptographic setting, these noisy channels must satisfy very strong security requirements. Unfair noisy channels, introduced by Damgaard, Kilian and Salvail [Eurocrypt '99], reduce these limitations: They give the adversary an unfair advantage over the honest player, and therefore weaken the security requirements on the noisy channel. However, this model still has many shortcomings: For example, the adversary's advantage is only allowed to have a very special form, and no error is allowed in the implementation. In this paper we generalize the idea of unfair noisy channels. We introduce two new models of cryptographic noisy channels that we call the weak erasure channel and the weak binary symmetric channel, and show how they can be used to implement oblivious transfer. Our models are more general and use much weaker assumptions than unfair noisy channels, which makes implementation a more realistic prospect.
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• Collapse and Revival of Entanglement between Qubits Interacting via a Quantum Bus
C. E. A. Jarvis, D. A. Rodrigues, B. L. Györffy, T. P. Spiller, A. J. Short, J. F. Annett
12 September 2008

Abstract:
We study the dynamics of the Jaynes-Cummings Model for two level systems (or qubits) interacting with a quantized single mode electromagnetic cavity (or quantum bus'). We show that there is a time in between the collapse and revival of Rabi oscillations when the state of the qubit sub-system, $| \psi\_{attractor}$, is largely independent of its initial state. This generalizes to many qubits the discovery by Gea-Banacloche for the one qubit case. The qubits in such attractor' states are not entangled either with the field or among themselves, even if they were in the initial state. Subsequently the entanglement between the qubits revives. Finally, it is argued that the collapse and revival of entanglement and the persistence of non-classicality' is a generic feature of multiple qubits interacting via a quantum bus'.
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• Instantaneous Quantum Computation
Dan Shepherd, Michael J. Bremner
05 September 2008

Abstract:
We examine theoretic architectures and an abstract model for a restricted class of quantum computation, called here instantaneous quantum computation because it allows for essentially no temporal structure within the quantum dynamics. Using the theory of binary matroids, we argue that the paradigm is rich enough to enable sampling from probability distributions that cannot, classically, be sampled from efficiently and accurately. This paradigm also admits simple interactive proof games that may convince a skeptic of the existence of truly quantum effects. Furthermore, these effects can be created using significantly fewer qubits than are required for running Shor's Algorithm.
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• Maximal Sets of Mutually Unbiased Quantum States in Dimension Six
Stephen Brierley, Stefan Weigert
12 August 2008
Phys. Rev. A 78, 042312 (2008)

Abstract:
We study sets of pure states in a Hilbert space of dimension d which are mutually unbiased (MU), that is, the squares of the moduli of their scalar products are equal to zero, one, or 1/d. These sets will be called a MU constellation, and if four MU bases were to exist for d=6, they would give rise to 35 different MU constellations. Using a numerical minimisation procedure, we are able to identify only 18 of them in spite of extensive searches. The missing MU constellations provide the strongest numerical evidence so far that no seven MU bases exist in dimension six.
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• Statistical Security Conditions for Two-Party Secure Function Evaluation
Claude Crépeau, Jürg Wullschleger
11 August 2008
Proceedings of ICITS 2008

Abstract:
To simplify proofs in information-theoretic security, the standard security definition of two-party secure function evaluation based on the real/ideal model paradigm is often replaced by an information-theoretic security definition. At EUROCRYPT 2006, we showed that most of these definitions had some weaknesses, and presented new information-theoretic conditions that were equivalent to a simulation-based definition in the real/ideal model. However, there we only considered the perfect case, where the protocol is not allowed to make any error, which has only limited applications. We generalize these results to the statistical case, where the protocol is allowed to make errors with a small probability. Our results are based on a new measure of information that we call the statistical information, which may be of independent interest.
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• Understanding photonic quantum-logic gates: The road to fault tolerance
Till J. Weinhold, Alexei Gilchrist, Kevin J. Resch, Andrew C. Doherty, Jeremy L. O'Brien, Geoffrey J. Pryde, Andrew G. White
07 August 2008

Abstract:
Fault-tolerant quantum computing requires gates which function correctly despite the presence of errors, and are scalable if the error probability-per-gate is below a threshold value. To date, no method has been described for calculating this probability from measurements on a gate. Here we introduce a technique enabling quantitative benchmarking of quantum-logic gates against fault-tolerance thresholds for any architecture. We demonstrate our technique experimentally using a photonic entangling-gate. The relationship between experimental errors and their quantum logic effect is non-trivial: revealing this relationship requires a comprehensive theoretical model of the quantum-logic gate. We show the first such model for any architecture, and find multi-photon emission--a small effect previously regarded as secondary to mode-mismatch--to be the dominant source of logic error. We show that reducing this will move photonic quantum computing to within striking distance of fault-tolerance.
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• Quantum Cellular Automata
K. Wiesner
06 August 2008

Abstract:
Quantum cellular automata (QCA) are reviewed, including early and more recent proposals. QCA are a generalization of (classical) cellular automata (CA) and in particular of reversible CA. The latter are reviewed shortly. An overview is given over early attempts by various authors to define one-dimensional QCA. These turned out to have serious shortcomings which are discussed as well. Various proposals subsequently put forward by a number of authors for a general definition of one- and higher-dimensional QCA are reviewed and their properties such as universality and reversibility are discussed.
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• Tamper-resistant encryption of quantum information
Andris Ambainis, Jan Bouda, Andreas Winter
05 August 2008

Abstract:
We introduce the notion of \emph{tamper resistance} of a quantum state encryption scheme (in dimension $d$): in addition to the requirement that an adversary cannot learn information about the state, here we demand that no controlled modification of the encrypted state can be effected.
We show that such a scheme is equivalent to a \emph{unitary 2-design} [Dankert \emph{et al.}], as opposed to normal encryption which is unitary 1-design. Our other main results include a new proof of the lower bound of $(d^2-1)^2+1$ on the number of unitaries in a 2-design [Gross \emph{et al.}], which lends itself to a generalization to approximate 2-design. Furthermore, while in each dimension there is a unitary 2-design with $\leq d^5$ elements, we show that there are approximate 2-designs with $O(\epsilon^{-2} d^4 \log d)$ elements.
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• Counterexamples to the maximal p-norm multiplicativity conjecture for all p > 1
Patrick Hayden, Andreas Winter
31 July 2008

Abstract:
For all p > 1, we demonstrate the existence of quantum channels with non-multiplicative maximal output p-norms. Equivalently, for all p >1, the minimum output Renyi entropy of order p of a quantum channel is not additive. The violations found are large; in all cases, the minimum output Renyi entropy of order p for a product channel need not be significantly greater than the minimum output entropy of its individual factors. Since p=1 corresponds to the von Neumann entropy, these counterexamples demonstrate that if the additivity conjecture of quantum information theory is true, it cannot be proved as a consequence of any channel-independent guarantee of maximal p-norm multiplicativity. We also show that a class of channels previously studied in the context of approximate encryption lead to counterexamples for all p > 2.
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• Composable Security in the Bounded-Quantum-Storage Model
Stephanie Wehner, Jürg Wullschleger
07 July 2008
Proceedings of ICALP 2008

Abstract:
We present a simplified framework for proving sequential composability in the quantum setting. In particular, we give a new, simulation-based, definition for security in the bounded-quantum-storage model, and show that this definition allows for sequential composition of protocols. Damgard et al. (FOCS '05, CRYPTO '07) showed how to securely implement bit commitment and oblivious transfer in the bounded-quantum-storage model, where the adversary is only allowed to store a limited number of qubits. However, their security definitions did only apply to the standalone setting, and it was not clear if their protocols could be composed. Indeed, we first give a simple attack that shows that these protocols are not composable without a small refinement of the model. Finally, we prove the security of their randomized oblivious transfer protocol in our refined model. Secure implementations of oblivious transfer and bit commitment then follow easily by a (classical) reduction to randomized oblivious transfer.
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• Error-Tolerant Combiners for Oblivious Primitives
Bartosz Przydatek, Jürg Wullschleger
07 July 2008
Proceedings of ICALP 2008

Abstract:
A robust combiner is a construction that combines several implementations of a primitive based on different assumptions, and yields an implementation guaranteed to be secure if at least some assumptions (i.e. sufficiently many but not necessarily all) are valid. In this paper we generalize this concept by introducing error-tolerant combiners, which in addition to protection against insecure implementations provide tolerance to functionality failures: an error-tolerant combiner guarantees a secure and correct implementation of the output primitive even if some of the candidates are insecure or faulty. We present simple constructions of error-tolerant robust combiners for oblivious linear function evaluation. The proposed combiners are also interesting in the regular (not error-tolerant) case, as the construction is much more efficient than the combiners known for oblivious transfer.
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• Cavity Enhanced Spin Measurement of an NV Centre in Diamond
A. Young, C.Y. Hu, L. Marseglia, J.P. Harrison, J.L. O'Brien, J.G. Rarity
02 July 2008

Abstract:
We propose a high efficiency high fidelity measurement of the ground state spin of a single NV center in diamond, using the effects of cavity quantum electrodynamics. The scheme we propose is based in the one dimensional atom or Purcell regime, removing the need for high Q cavities that are challenging to fabricate. The ground state of the NV center consists of three spin levels $^{3}A_{(m=0)}$ and $^{3}A_{(m=\pm1)}$ (the $\pm1$ states are near degenerate in zero field). These two states can undergo transitions to the excited ($^{3}E$) state, with an energy difference of $\approx7-10$ $\mu$eV between the two. By choosing the correct Q factor, this small detuning between the two transitions results in a dramatic change in the intensity of reflected light. We show the change in reflected intensity can allow us to read out the ground state spin using a low intensity laser with an error rate of $\approx5.5\times10^{-3}$, when realistic cavity and experimental parameters are considered. Since very low levels of light are used to probe the state of the spin we limit the number of florescence cycles, thereby limiting the non spin preserving transitions through the intermediate singlet state $^{1}A$.
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• Entanglement and intra-molecular cooling in biological systems? - A quantum thermodynamic perspective
Hans J. Briegel, Sandu Popescu
30 June 2008

Abstract:
We discuss the possibility of existence of entanglement in biological systems. Our arguments centre on the fact that biological systems are thermodynamic open driven systems far from equilibrium. In such systems error correction can occur which may maintain entanglement despite high levels of de-coherence. We also discuss the possibility of cooling (classical or quantum) at molecular level.
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• Quantum algorithms for shifted subset problems
Ashley Montanaro
23 June 2008
Quantum Information & Computation vol. 9 no. 5&6, pp. 500-512, 2009

Abstract:
We consider a recently proposed generalisation of the abelian hidden subgroup problem: the shifted subset problem. The problem is to determine a subset S of some abelian group, given access to quantum states of the form |S+x>, for some unknown shift x. We give quantum algorithms to find Hamming spheres and other subsets of the boolean cube {0,1}^n. The algorithms have time complexity polynomial in n and give rise to exponential separations from classical computation.
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• Non-secret correlations can be used to distribute secrecy
Joonwoo Bae, Toby Cubitt, Antonio Acin
11 June 2008

Abstract:
A counter-intuitive result in entanglement theory was shown in [PRL 91 037902 (2003)], namely that entanglement can be distributed by sending a separable state through a quantum channel. In this work, following an analogy between the entanglement and secret key distillation scenarios, we derive its classical analog: secrecy can be distributed by sending non-secret correlations through a private channel. This strengthens the close relation between entanglement and secrecy.
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• Space-QUEST: Experiments with quantum entanglement in space
Rupert Ursin, Thomas Jennewein, Johannes Kofler, Josep M. Perdigues, Luigi Cacciapuoti, Clovis J. de Matos, Markus Aspelmeyer, Alejandra Valencia, Thomas Scheidl, Alessandro Fedrizzi, Antonio Acin, Cesare Barbieri, Giuseppe Bianco, Caslav Brukner, Jose Capmany, Sergio Cova, Dirk Giggenbach, Walter Leeb, Robert H. Hadfield, Raymond Laflamme, Norbert Lutkenhaus, Gerard Milburn, Momtchil Peev, Timothy Ralph, John Rarity, Renato Renner, Etienne Samain, Nikolaos Solomos, Wolfgang Tittel, Juan P. Torres, Morio Toyoshima, Arturo Ortigosa-Blanch, Valerio Pruneri, Paolo Villoresi, Ian Walmsley, Gregor Weihs, Harald Weinfurter, Marek Zukowski, Anton Zeilinger
06 June 2008
IAC Proceedings A2.1.3 (2008)

Abstract:
The European Space Agency (ESA) has supported a range of studies in the field of quantum physics and quantum information science in space for several years, and consequently we have submitted the mission proposal Space-QUEST (Quantum Entanglement for Space Experiments) to the European Life and Physical Sciences in Space Program. We propose to perform space-to-ground quantum communication tests from the International Space Station (ISS). We present the proposed experiments in space as well as the design of a space based quantum communication payload.
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• Exact universality from any entangling gate without inverses
Aram W. Harrow
05 June 2008
Q. Inf. Comp. Vol.8, No.8&9, pp. 715--721, 2008

Abstract:
This note proves that arbitrary local gates together with any entangling bipartite gate V are universal. Previously this was known only when access to both V and V^{-1} was given, or when approximate universality was demanded.
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• New Monotones and Lower Bounds in Unconditional Two-Party Computation
Stefan Wolf, Jürg Wullschleger
01 June 2008
IEEE Transactions on Information Theory, vol. 54, no. 6, pp. 2792-2795, 2008

Abstract:
Since oblivious transfer, a primitive of paramount importance in secure two- and multi-party computation, cannot be realized in an unconditionally secure way for both parties from scratch, reductions to weak information-theoretic primitives as well as between different variants of the functionality are of great interest. In this context, we introduce various monotones-quantities that cannot be increased by any protocol-and use them to derive lower bounds on the possibility and efficiency of such reductions.
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• A deterministic optical quantum computer using photonic modules
A. M. Stephens, Z. W. E. Evans, S. J. Devitt, A. D. Greentree, A. G. Fowler, W. J. Munro, J. L. O'Brien, Kae Nemoto, L. C. L. Hollenberg
26 May 2008
PRA 78, 032318 (2008)

Abstract:
The optical quantum computer is one of the few experimental systems to have demonstrated small scale quantum information processing. Making use of cavity quantum electrodynamics approaches to operator measurements, we detail an optical network for the deterministic preparation of arbitrarily large two-dimensional cluster states. We show that this network can form the basis of a large scale deterministic optical quantum computer that can be fabricated entirely on chip.
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• Superpolynomial speedups based on almost any quantum circuit
Sean Hallgren, Aram W. Harrow
02 May 2008
Proc. of ICALP 2008, LNCS 5125, pp. 782-795

Abstract:
The first separation between quantum polynomial time and classical bounded-error polynomial time was due to Bernstein and Vazirani in 1993. They first showed a O(1) vs. Omega(n) quantum-classical oracle separation based on the quantum Hadamard transform, and then showed how to amplify this into a n^{O(1)} time quantum algorithm and a n^{Omega(log n)} classical query lower bound.
We generalize both aspects of this speedup. We show that a wide class of unitary circuits (which we call _dispersing_ circuits) can be used in place of Hadamards to obtain a O(1) vs. Omega(n) separation. The class of dispersing circuits includes all quantum Fourier transforms (including over nonabelian groups) as well as nearly all sufficiently long random circuits. Second, we give a general method for amplifying quantum-classical separations that allows us to achieve a n^{O(1)} vs. n^{Omega(log n)} separation from any dispersing circuit.
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• A non-distillability criterion for secret correlations
Lluis Masanes, Andreas Winter
30 April 2008
Quantum Information and Computation, Vol.10, No.1&2, pp0152 (2009)

Abstract:
Within entanglement theory there are criteria which certify that some quantum states cannot be distilled into pure entanglement. An example is the positive partial transposition criterion. Here we present, for the first time, the analogous thing for secret correlations. We introduce a computable criterion which certifies that a probability distribution between two honest parties and an eavesdropper cannot be (asymptotically) distilled into a secret key. Nothing is known about the existence of non-distillable correlations with positive secrecy cost, also known as bound information. This criterion may be the key for finding such correlations, but we ignore whether this is the case. If this is not the case, and hence the criterion is useless, this implies a very interesting result: Any correlation with positive secrecy cost can increase the secrecy content of another. In other words, all correlations with positive secrecy cost constitute a useful resource.
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• Spin-flip and spin-conserving optical transitions of the nitrogen-vacancy centre in diamond
Tamarat Ph, Manson NB, Harrison JP, McMurtrie RL, Nizovtsev A, Santori C, Beausoleil RG, Neumann P, Gaebel T, Jelezko F, Hemmer P, Wrachtrup J
30 April 2008
NEW JOURNAL OF PHYSICS

Abstract:
We map out the first excited state sublevel structure of single nitrogen-vacancy (NV) colour centres in diamond. The excited state is an orbital doublet where one branch supports an efficient cycling transition, while the other can simultaneously support fully allowed optical Raman spin-flip transitions. This is crucial for the success of many recently proposed quantum information applications of the NV defects. We further find that an external electric field can be used to completely control the optical properties of a single centre. Finally, a group theoretical model is developed that explains the observations and provides good physical understanding of the excited state structure.
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• Matchgates and classical simulation of quantum circuits
Richard Jozsa, Akimasa Miyake
28 April 2008
Proc. Roy. Soc. (Lond.) A 464, p3089-3106 (Dec 2008)

Abstract:
Let G(A,B) denote the 2-qubit gate which acts as the 1-qubit SU(2) gates A and B in the even and odd parity subspaces respectively, of two qubits. Using a Clifford algebra formalism we show that arbitrary uniform families of circuits of these gates, restricted to act only on nearest neighbour (n.n.) qubit lines, can be classically efficiently simulated. This reproduces a result originally proved by Valiant using his matchgate formalism, and subsequently related by others to free fermionic physics. We further show that if the n.n. condition is slightly relaxed, to allowing the same gates to act only on n.n. and next-n.n. qubit lines, then the resulting circuits can efficiently perform universal quantum computation. From this point of view, the gap between efficient classical and quantum computational power is bridged by a very modest use of a seemingly innocuous resource (qubit swapping). We also extend the simulation result above in various ways. In particular, by exploiting properties of Clifford operations in conjunction with the Jordan-Wigner representation of a Clifford algebra, we show how one may generalise the simulation result above to provide further classes of classically efficiently simulatable quantum circuits, which we call Gaussian quantum circuits.
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• Quantum computing using shortcuts through higher dimensions
B. P. Lanyon, M. Barbieri, M. P. Almeida, T. Jennewein, T. C. Ralph, K. J. Resch, G. J. Pryde, J. L. O'Brien, A. Gilchrist, A. G. White
03 April 2008

Abstract:
Quantum computation offers the potential to solve fundamental yet otherwise intractable problems across a range of active fields of research. Recently, universal quantum-logic gate sets - the building blocks for a quantum computer - have been demonstrated in several physical architectures. A serious obstacle to a full-scale implementation is the sheer number of these gates required to implement even small quantum algorithms. Here we present and demonstrate a general technique that harnesses higher dimensions of quantum systems to significantly reduce this number, allowing the construction of key quantum circuits with existing technology. We are thereby able to present the first implementation of two key quantum circuits: the three-qubit Toffoli and the two-qubit controlled-unitary. The gates are realised in a linear optical architecture, which would otherwise be absolutely infeasible with current technology.
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• Beating the standard quantum limit: Phase super-sensitivity of N-photon interferometers
Ryo Okamoto, Holger F. Hofmann, Tomohisa Nagata, Jeremy L. O'Brien, Keiji Sasaki, Shigeki Takeuchi
02 April 2008

Abstract:
Quantum metrology promises greater sensitivity for optical phase measurements than could ever be achieved classically. Here we present a theory of the phase sensitivity for the general case where the detection probability is given by an $N$ photon interference fringe. We find that the phase sensitivity has a complex dependence on both the intrinsic efficiency of detection $\eta$ and the interference fringe visibility $V$. Most importantly, the phase that gives maximum phase sensitivity is in general not the same as the phase at which the slope of the interference fringe is a maximum, as has previously been assumed. We determine the parameter range where quantum enhanced sensitivity can be achieved. In order to illustrate these theoretical results, we perform a four photon experiment with $\eta=3/4$ and $V=82\pm6$% (an extension of our previous work [Science \textbf{316}, 726 (2007)]) and find a phase sensitivity 1.3 times greater than the standard quantum limit at a phase different to that which gives maximum slope of the interference fringe.
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• Classical and Quantum Tensor Product Expanders
M. B. Hastings, A. W. Harrow
02 April 2008
QIC 9, 336 (2009)

Abstract:
We introduce the concept of quantum tensor product expanders. These are expanders that act on several copies of a given system, where the Kraus operators are tensor products of the Kraus operator on a single system. We begin with the classical case, and show that a classical two-copy expander can be used to produce a quantum expander. We then discuss the quantum case and give applications to the Solovay-Kitaev problem. We give probabilistic constructions in both classical and quantum cases, giving tight bounds on the expectation value of the largest nontrivial eigenvalue in the quantum case.
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• A communication-efficient nonlocal measurement with application to communication complexity and bipartite gate capacities
Aram W. Harrow, Debbie W. Leung
21 March 2008

Abstract:
Two dual questions in quantum information theory are to determine the communication cost of simulating a bipartite unitary gate, and to determine their communication capacities. We present a bipartite unitary gate with two surprising properties: 1) simulating it with the assistance of unlimited EPR pairs requires far more communication than with a better choice of entangled state, and 2) its communication capacity is far lower than its capacity to create entanglement. This suggests that 1) unlimited EPR pairs are not the most general model of entanglement assistance for two-party communication tasks, and 2) the entangling and communicating abilities of a unitary interaction can vary nearly independently. The technical contribution behind these results is a communication-efficient protocol for measuring whether an unknown shared state lies in a specified rank-one subspace or its orthogonal complement.
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• Simulation of partial entanglement with no-signaling resources
Nicolas Brunner, Nicolas Gisin, Sandu Popescu, Valerio Scarani
18 March 2008
Phys. Rev. A 78, 052111 (2008)

Abstract:
With the goal of gaining a deeper understanding of quantum non-locality, we decompose quantum correlations into more elementary non-local correlations. In particular we present two models for simulating the correlations of partially entangled states of two qubits without communication, hence using only non-signaling resources. The crucial role of the quantum marginals is discussed.
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• Optical Quantum Computing
Jeremy L. O'Brien
12 March 2008
Science, 318, 1467 (2007)

Abstract:
In 2001 all-optical quantum computing became feasible with the discovery that scalable quantum computing is possible using only single photon sources, linear optical elements, and single photon detectors. Although it was in principle scalable, the massive resource overhead made the scheme practically daunting. However, several simplifications were followed by proof-of-principle demonstrations, and recent approaches based on cluster states or error encoding have dramatically reduced this worrying resource overhead, making an all-optical architecture a serious contender for the ultimate goal of a large-scale quantum computer. Key challenges will be the realization of high-efficiency sources of indistinguishable single photons, low-loss, scalable optical circuits, high efficiency single photon detectors, and low-loss interfacing of these components.
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• Random Quantum Circuits are Approximate 2-designs
Aram Harrow, Richard Low
15 February 2008
Comm. Math. Phys., 291(1):257-302, 2009

Abstract:
Given a universal gate set on two qubits, it is well known that applying random gates from the set to random pairs of qubits will eventually yield an approximately Haar-distributed unitary. However, this requires exponential time. We show that random circuits of only polynomial length will approximate the first and second moments of the Haar distribution, thus forming approximate 1- and 2-designs. Previous constructions required longer circuits and worked only for specific gate sets. As a corollary of our main result, we also improve previous bounds on the convergence rate of random walks on the Clifford group.
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• An All Optical Fibre Quantum Controlled-NOT Gate
Alex S. Clark, Jeremie Fulconis, John G. Rarity, William J. Wadsworth, Jeremy L. O'Brien
13 February 2008
Phys. Rev. A 79, 030303(R) (2009)

Abstract:
We report the first experimental demonstration of an optical controlled-NOT gate constructed entirely in fibre. We operate the gate using two heralded optical fibre single photon sources and find an average logical fidelity of 90% and an average process fidelity of 0.83<F<0.91. On the basis of a simple model we are able to conclude that imperfections are primarily due to the photon sources, meaning that the gate itself works with very high fidelity.
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• The structure of degradable quantum channels
Toby Cubitt, Mary-Beth Ruskai, Graeme Smith
12 February 2008
J. Math. Phys. 49, 102104 (2008)

Abstract:
Degradable quantum channels are among the only channels whose quantum and private classical capacities are known. As such, determining the structure of these channels is a pressing open question in quantum information theory. We give a comprehensive review of what is currently known about the structure of degradable quantum channels, including a number of new results as well as alternate proofs of some known results. In the case of qubits, we provide a complete characterization of all degradable channels with two dimensional output, give a new proof that a qubit channel with two Kraus operators is either degradable or anti-degradable and present a complete description of anti-degradable unital qubit channels with a new proof.
For higher output dimensions we explore the relationship between the output and environment dimensions ($d_B$ and $d_E$ respectively) of degradable channels. For several broad classes of channels we show that they can be modeled with a environment that is "small" in the sense $d_E \leq d_B$. Perhaps surprisingly, we also present examples of degradable channels with large'' environments, in the sense that the minimal dimension $d_E > d_B$. Indeed, one can have $d_E > \tfrac{1}{4} d_B^2$.
In the case of channels with diagonal Kraus operators, we describe the subclass which are complements of entanglement breaking channels. We also obtain a number of results for channels in the convex hull of conjugations with generalized Pauli matrices. However, a number of open questions remain about these channels and the more general case of random unitary channels.
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• Efficiently contractable quantum circuits cannot produce much entanglement
Nadav Yoran
11 February 2008

Abstract:
We show a similarity between two different classical simulation methods for measurement based quantum computation -- one relying on a low entanglement (tree tensor network) representation of the computer's state, and the other a tensor contraction method based on the topology of the graph state. We use this similarity to show that any quantum circuit that can be efficiently simulated via tensor contraction cannot produce much entanglement.
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• Silica-on-Silicon Waveguide Quantum Circuits
Alberto Politi, Martin J. Cryan, John G. Rarity, Siyuan Yu, Jeremy L. O'Brien
04 February 2008
Science 27 March (2008) (10.1126/science.1155441)

Abstract:
Quantum technologies based on photons are anticipated in the areas of information processing, communication, metrology, and lithography. While there have been impressive proof-of-principle demonstrations in all of these areas, future technologies will likely require an integrated optics architecture for improved performance, miniaturization and scalability. We demonstrated high- fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference with a visibility of 94.8(5)%; a controlled-NOT gate with logical basis fidelity of 94.3(2)%; and a path entangled state of two photons with fidelity >92%.
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• Quantum communication beyond the localization length in disordered spin chains
Jonathan Allcock, Noah Linden
01 February 2008
Phys. Rev. Lett. 102, 110501 (2009)

Abstract:
We study the effects of localization on quantum state transfer in spin chains. We show how to use quantum error correction and multiple parallel spin chains to send a qubit with high fidelity over arbitrary distances; in particular distances much greater than the localization length of the chain.
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• Pure-state transformations and catalysis under operations that completely preserve positivity of partial transpose
William Matthews, Andreas Winter
29 January 2008
Phys. Rev. A 78, 012317 (2008)

Abstract:
Motivated by the desire to better understand the class of quantum operations on bipartite systems that preserve positivity of partial transpose (PPT operations) and its relation to the class LOCC (local operations and classical communication), we present some results on deterministic bipartite pure state transformations by PPT operations. Restricting our attention to the case where we start with a rank K maximally entangled state, we give a necessary condition for transforming it into a given pure state, which we show is also sufficient when K is two and the final state has Schmidt rank three. We show that it is sufficient for all K and all final states provided a conjecture about a certain family of semidefinite programs is true. We also demonstrate that the phenomenon of catalysis can occur under PPT operations and that, unlike LOCC catalysis, a maximally entangled state can be a catalyst. Finally, we give a necessary and sufficient condition for the possibility of transforming a rank K maximally entangled state to an arbitrary pure state by PPT operations assisted by some maximally entangled catalyst.
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• Experimental Quantum Process Discrimination
Anthony Laing, Terry Rudolph, Jeremy L. O'Brien
25 January 2008
Phys. Rev. Lett. 102, 160502 (2009)

Abstract:
The problem of discriminating between unknown processes chosen from a finite set is experimentally shown to be possible even in the case of non-orthogonal processes. We demonstrate unambiguous deterministic quantum process discrimination (QPD) of non-orthogonal processes using properties of entanglement, additional known unitaries, or higher dimensional systems. Single qubit, qutrit and qudit ($d$=10) measurement and unitary processes acting on photons are discriminated with a confidence of $>97%$ in all cases.
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• Entanglement-Enhanced Quantum Key Distribution
Olli Ahonen, Mikko Mottonen, Jeremy O'Brien
28 December 2007
Physical Review A 78, 032314 (2008)

Abstract:
We present and analyze a quantum key distribution protocol based on sending entangled multi-qubit states instead of single-qubit states as in the trail-blazing scheme by Bennett and Brassard (BB84). Since the qubits are sent individually, an eavesdropper is limited to access them one by one. In an intercept-resend attack on entangled two-qubit states, this fundamental restriction is shown to reduce the information of the eavesdropper to be less than one third compared with BB84. Strikingly, the information gain can be made to vanish if eavesdropping is applied to only one of the pairwise entangled qubits. For entangled states of many qubits, our protocol is expected to show even more pronounced, potentially exponential, advantages.
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• Counterexamples to additivity of minimum output p-Renyi entropy for p close to 0
Toby Cubitt, Aram W. Harrow, Debbie Leung, Ashley Montanaro, Andreas Winter
24 December 2007
Comm. Math. Phys Volume 284, Number 1, November 2008

Abstract:
Complementing recent progress on the additivity conjecture of quantum information theory, showing that the minimum output p-Renyi entropies of channels are not generally additive for p>1, we demonstrate here by a careful random selection argument that also at p=0, and consequently for sufficiently small p, there exist counterexamples.
An explicit construction of two channels from 4 to 3 dimensions is given, which have non-multiplicative minimum output rank; for this pair of channels, numerics strongly suggest that the p-Renyi entropy is non-additive for all p < 0.12. We conjecture however that violations of additivity exist for all p<1.
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• Unbounded Error Quantum Query Complexity
Ashley Montanaro, Harumichi Nishimura, Rudy Raymond
11 December 2007
In Proc. ISAAC'08 (International Symposium on Algorithms and Computation)

Abstract:
This work studies the quantum query complexity of Boolean functions in a scenario where it is only required that the query algorithm succeeds with a probability strictly greater than 1/2. We show that, just as in the communication complexity model, the unbounded error quantum query complexity is exactly half of its classical counterpart for any (partial or total) Boolean function. Moreover, we show that the "black-box" approach to convert quantum query algorithms into communication protocols by Buhrman-Cleve-Wigderson [STOC'98] is optimal even in the unbounded error setting.
We also study a setting related to the unbounded error model, called the weakly unbounded error setting, where the cost of a query algorithm is given by q+log(1/2(p-1/2)), where q is the number of queries made and p>1/2 is the success probability of the algorithm. In contrast to the case of communication complexity, we show a tight Theta(log n) separation between quantum and classical query complexity in the weakly unbounded error setting for a partial Boolean function. We also show the asymptotic equivalence between them for some well-studied total Boolean functions.
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• Random quantum codes from Gaussian ensembles and an uncertainty relation
Patrick Hayden, Peter W. Shor, Andreas Winter
07 December 2007
Open Syst. Inf. Dyn. 15 (2008) 71-89

Abstract:
Using random Gaussian vectors and an information-uncertainty relation, we give a proof that the coherent information is an achievable rate for entanglement transmission through a noisy quantum channel. The codes are random subspaces selected according to the Haar measure, but distorted as a function of the sender's input density operator. Using large deviations techniques, we show that classical data transmitted in either of two Fourier-conjugate bases for the coding subspace can be decoded with low probability of error. A recently discovered information-uncertainty relation then implies that the quantum mutual information for entanglement encoded into the subspace and transmitted through the channel will be high. The monogamy of quantum correlations finally implies that the environment of the channel cannot be significantly coupled to the entanglement, and concluding, which ensures the existence of a decoding by the receiver.
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• Multiple-time states and multiple-time measurements in quantum mechanics
Y. Aharonov, S. Popescu, J. Tollaksen, L. Vaidman
04 December 2007

Abstract:
We discuss experimental situations that consist of multiple preparation and measurement stages. This leads us to a new approach to quantum mechanics. In particular, we introduce the idea of multi-time quantum states which are the appropriate tools for describing these experimental situations. We also describe multi-time measurements and discuss their relation to multi-time states. A consequence of our new formalism is to put states and operators on an equal footing. Finally we discuss the implications of our new approach to quantum mechanics for the problem of the flow of time.
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• Quantum evolution from a snapshot
Michael M. Wolf, Jens Eisert, Toby S. Cubitt, J. Ignacio Cirac
21 November 2007
to appear in Phys. Rev. Lett.

Abstract:
We investigate what a snapshot of a quantum evolution - a quantum channel reflecting open system dynamics - reveals about the underlying continuous time evolution. Remarkably, from such a snapshot, and without imposing additional assumptions, it is possible to decide whether or not a channel is consistent with a time (in)dependent Markovian evolution, for which we provide computable necessary and sufficient criteria. Based on these, a computable measure of `Markovianity' is introduced which quantifies the Markovian part of a quantum channel. We discuss the consistency with Markovian dynamics as encountered in quantum process tomography for physical non-Markovian processes. The results clarify the geometry of the set of quantum channels with respect to being solutions of time (in)dependent master equations or (in)divisible channels.
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• A lower bound on the probability of error in quantum state discrimination
Ashley Montanaro
14 November 2007
In Proc. IEEE Information Theory Workshop 2008

Abstract:
We give a lower bound on the probability of error in quantum state discrimination in terms of a weighted sum of the pairwise fidelities of the states to be distinguished.
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• Entangling photons using a charged quantum dot in a microcavity
C.Y. Hu, W.J. Munro, J.G. Rarity
12 November 2007
Phys. Rev. B 78, 125318 (2008)

Abstract:
We present two novel schemes to generate photon polarization entanglement via single electron spins confined in charged quantum dots inside microcavities. One scheme is via entangled remote electron spins followed by negatively-charged exciton emissions, and another scheme is via a single electron spin followed by the spin state measurement. Both schemes are based on giant circular birefringence and giant Faraday rotation induced by a single electron spin in a microcavity. Our schemes are deterministic and can generate an arbitrary amount of multi-photon entanglement. Following similar procedures, a scheme for a photon-spin quantum interface is proposed.
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• On the Chernoff distance for asymptotic LOCC discrimination of bipartite quantum states
William Matthews, Andreas Winter
23 October 2007
to appear in Comm. Math. Phys.

Abstract:
Motivated by the recent discovery of a quantum Chernoff theorem for asymptotic state discrimination, we investigate the distinguishability of two bipartite mixed states under the constraint of local operations and classical communication (LOCC), in the limit of many copies. While for two pure states a result of Walgate et al. shows that LOCC is just as powerful as global measurements, data hiding states (DiVincenzo et al.) show that locality can impose severe restrictions on the distinguishability of even orthogonal states. Here we determine the optimal error probability and measurement to discriminate many copies of particular data hiding states (extremal d x d Werner states) by a linear programming approach. Surprisingly, the single-copy optimal measurement remains optimal for n copies, in the sense that the best strategy is measuring each copy separately, followed by a simple classical decision rule. We also put a lower bound on the bias with which states can be distinguished by separable operations.
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• Higher entropic uncertainty relations for anti-commuting observables
Stephanie Wehner, Andreas Winter
08 October 2007
Journal of Mathematical Physics, 49, 062105 (2008)

Abstract:
Uncertainty relations lie at the very core of quantum mechanics, and form the cornerstone of essentially all quantum cryptographic applications. In particular, they play an important role in cryptographic protocols in the bounded-quantum-storage model, where proving the security of all existing protocols ultimately reduces to bounding such relations. Yet, very little is known about such uncertainty relations for more than two measurements. Here, we prove optimal entropic uncertainty relations for anti-commuting binary observables for the Shannon entropy, and nearly optimal relations for the collision entropy. Our results have immediate applications to quantum cryptography.
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• Quantum expanders from any classical Cayley graph expander
Aram W. Harrow
10 September 2007
Q. Inf. Comp., vol. 8, no. 8/9, pp. 715-721, 2008.

Abstract:
We give a simple recipe for translating walks on Cayley graphs of a group G into a quantum operation on any irrep of G. Most properties of the classical walk carry over to the quantum operation: degree becomes the number of Kraus operators, the spectral gap becomes the gap of the quantum operation (viewed as a linear map on density matrices), and the quantum operation is efficient whenever the classical walk and the quantum Fourier transform on G are efficient. This means that using classical constant-degree constant-gap families of Cayley expander graphs on e.g. the symmetric group, we can construct efficient families of quantum expanders.
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• Entanglement of superconducting charge qubits by homodyne measurement
D. A. Rodrigues, C. E. A. Jarvis, B. L. Györffy, T. P. Spiller, J. F. Annett
22 August 2007
J. Phys.: Condens. Matter, 20, (2008), 075211

Abstract:
We present a scheme by which projective homodyne measurement of a microwave resonator can be used to generate entanglement between two superconducting charge qubits coupled to this resonator. The non-interacting qubits are initialised in a product of their ground states, the resonator is initialised in a coherent field state, and the state of the system is allowed to evolve under a rotating wave Hamiltonian. Making a homodyne measurement on the resonator at a given time projects the qubits into an state of the form (|gg> + exp(-i phi)|ee>)/sqrt(2). This protocol can produce states with a fidelity as high as required, with a probability approaching 0.5. Although the system described is one that can be used to display revival in the qubit oscillations, we show that the entanglement procedure works at much shorter timescales.
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• Giant optical Faraday rotation induced by a single-electron spin in a quantum dot: Applications to entangling remote spins via a single photon
C. Y. Hu, A. Young, J. L. O'Brien, W.J. Munro, J. G. Rarity
16 August 2007
Phys. Rev. B 78, 085307 (2008)

Abstract:
We propose a novel non-destructive method - giant Faraday rotation - to detect a single electron spin in a quantum dot inside a microcavity where negatively-charged exciton strongly couples to the cavity mode. Left- and right-circularly polarized light reflected from the cavity feels different phase shifts due to cavity quantum electrodynamics and the optical spin selection rule. This yields giant and tunable Faraday rotation which can be easily detected experimentally. Based on this spin-detection technique, a scalable scheme to create an arbitrary amount of entanglement between two or more remote spins via photons is proposed.
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• Beating the Standard Quantum Limit with Four Entangled Photons
Tomohisa Nagata, Ryo Okamoto, Jeremy L. O' Brien, Keiji Sasaki, Shigeki Takeuchi
13 August 2007
Science 316, 726-729 (2007)

Abstract:
Precision measurements are important across all fields of science. In particular, optical phase measurements can be used to measure distance, position, displacement, acceleration and optical path length. Quantum entanglement enables higher precision than would otherwise be possible. We demonstrate an optical phase measurement with an entangled four photon interference visibility greater than the threshold to beat the standard quantum limit--the limit attainable without entanglement. These results open the way for new high-precision measurement applications.
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• Codeword Stabilized Quantum Codes
Andrew Cross, Graeme Smith, John A. Smolin, Bei Zeng
09 August 2007
IEEE Trans. Info. Theory 55, 1, 433-438 (2009)

Abstract:
We present a unifying approach to quantum error correcting code design that encompasses additive (stabilizer) codes, as well as all known examples of nonadditive codes with good parameters. We use this framework to generate new codes with superior parameters to any previously known. In particular, we find ((10,18,3)), ((10,20,3)) and ((11,48,3)) codes.
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• Optical Quadbit One-way Quantum Computation
Jaewoo Joo, Peter L. Knight, Jeremy L. O'Brien, Terry Rudolph
01 August 2007
Phys. Rev. A 76, 052326 (2007)

Abstract:
We consider the possibility of performing linear optical quantum computation making use of extra photonic degrees of freedom. In particular we focus on the case where we use photons as quadbits. The basic 2-quadbit cluster state is a hyper-entangled state across polarization and two spatial mode degrees of freedom. We examine the non-deterministic methods whereby such states can be created from single photons and/or Bell pairs, and then give some mechanisms for performing higher-dimensional fusion gates.
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• Polarising, Rotating and Entangling Photonic Qutrits
B. P. Lanyon, T. J. Weinhold, N. K. Langford, J. L. O'Brien, K. J. Resch, A. Gilchrist, A. G. White
20 July 2007
Phys. Rev. Lett. 100, 060504 (2008)

Abstract:
The polarisation of a pair of photons in the same spatio-temporal mode represents a three-level quantum system, a qutrit. However, several of the key advantages of single photon polarisation qubits are lost when moving to this higher dimension. These advantages include easy arbitrary rotations, polarising elements and entangling operations. We present a technique based on measurement-induced nonlinearity that greatly extends the range of qutrit operations possible using linear optics. Furthermore, we use the nonlinearity to create the first qubit-qutrit entangled state which we fully characterise using quantum state tomography.
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• The maximum output p-norm of quantum channels is not multiplicative for any p>2
Andreas Winter
04 July 2007

Abstract:
We prove the claim made in the title by showing that approximately randomising maps of large dimension are counterexamples to the multiplicativity conjecture.
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• Complex weak values in quantum measurement
Richard Jozsa
28 June 2007
Phys. Rev. A vol 76, p044103 (2007)

Abstract:
In the weak measurement formalism of Y. Aharonov et al. the so-called weak value A_w of any observable A is generally a complex number. We derive a physical interpretation of its value in terms of the shift in the measurement pointer's mean position and mean momentum. In particular we show that the mean position shift contains a term jointly proportional to the imaginary part of the weak value and the rate at which the pointer is spreading in space as it enters the measurement interaction.
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• The Photonic Module: an on-demand resource for photonic entanglement
Simon J. Devitt, Andrew D. Greentree, Jeremy O'Brien, Lloyd C.L. Hollenberg
18 June 2007
Phys. Rev. A. 76, 052312 (2007)

Abstract:
Entanglement is the enabling resource for a new generation of advanced technology. Photonic entanglement is the most versatile, finding applications across the fields of quantum communication, lithography, metrology, cryptography and large scale quantum computation. However there are as yet no convenient, on-demand devices for rapidly generating large-multi photon entangled states from single photon sources. Here we introduce a new device called the "photonic module" to address this need. Allowing, for the first time, the rapid deterministic preparation of a large class of entangled states with an application independent, "plug and play" device, with significant flexibility to generate entangled states for all major quantum computation and communication applications. The heart of the module is a controllable atom/cavity system, which when combined with a linear optical network, entangles photons quickly and deterministically. Recent experimental advances in cavity QED suggest that construction of this device is imminently possible. The module offers great potential for the integration of advanced theoretical and experimental techniques in quantum atom optics, optical computation and photonics providing a new enabling resource for quantum information science.
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• Sequential weak measurement
Graeme Mitchison, Richard Jozsa, Sandu Popescu
12 June 2007
Phys. Rev. A 76, 062105 (2007)

Abstract:
The notion of weak measurement provides a formalism for extracting information from a quantum system in the limit of vanishing disturbance to its state. Here we extend this formalism to the measurement of sequences of observables. When these observables do not commute, we may obtain information about joint properties of a quantum system that would be forbidden in the usual strong measurement scenario. As an application, we provide a physically compelling characterisation of the notion of counterfactual quantum computation.
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• Classical simulability and the significance of modular exponentiation in Shor's algorithm
Nadav Yoran, Anthony J. Short
07 June 2007
Phys. Rev. A 76, 060302(R) (2007).

Abstract:
We show that a classical algorithm efficiently simulating the modular exponentiation circuit, for certain product state input and with measurements in a general product state basis at the output, can efficiently simulate Shor's factoring algorithm. This is done by using the notion of the semi-classical Fourier transform due to Griffith and Niu, and further discussed in the context of Shor's algorithm by Browne.
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• On the dimension of subspaces with bounded Schmidt rank
T. S. Cubitt, A. Montanaro, A. Winter
06 June 2007
J. Math. Phys. 49, 022107 (2008)

Abstract:
We consider the question of how large a subspace of a given bipartite quantum system can be when the subspace contains only highly entangled states. This is motivated in part by results of Hayden et al., which show that in large d x d--dimensional systems there exist random subspaces of dimension almost d^2, all of whose states have entropy of entanglement at least log d - O(1). It is also related to results due to Parthasarathy on the dimension of completely entangled subspaces, which have connections with the construction of unextendible product bases. Here we take as entanglement measure the Schmidt rank, and determine, for every pair of local dimensions dA and dB, and every r, the largest dimension of a subspace consisting only of entangled states of Schmidt rank r or larger. This exact answer is a significant improvement on the best bounds that can be obtained using random subspace techniques. We also determine the converse: the largest dimension of a subspace with an upper bound on the Schmidt rank. Finally, we discuss the question of subspaces containing only states with Schmidt equal to r.
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• The private classical capacity with a symmetric side channel and its application to quantum cryptography
Graeme Smith
29 May 2007
Phys. Rev. A 78, 022306 (2008)

Abstract:
We study the symmetric-side-channel-assisted private capacity of a quantum channel, for which we provide a single-letter formula. This capacity is additive, convex, and, for degradable channels, equal to the unassisted private capacity. While a channel's (unassisted) capacity for for private classical communication may be strictly larger than its quantum capacity, we will show that these capacities are equal for degradable channels, thus demonstrating the equivalence of privacy and quantum coherence in this context. We use these ideas to find new bounds on the key rate of quantum key distribution protocols with one-way classical post-processing. For the Bennett-Brassard-84 (BB84) protocol, our results demonstrate that collective attacks are strictly stronger than individual attacks.
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• Entanglement and separability of quantum harmonic oscillator systems at finite temperature
Janet Anders, Andreas Winter
22 May 2007
Quantum Information & Computation, 8 (3 & 4), pp 0245-0262 (2008)

Abstract:
In the present paper we study the entanglement properties of thermal (a.k.a. Gibbs) states of finite quantum harmonic oscillator systems as functions of the Hamiltonian and the temperature. We prove the physical intuition that at sufficiently high temperatures the thermal state becomes fully separable and we deduce bounds on the critical temperature at which this happens. We show that the bound becomes tight for a wide class of Hamiltonians with sufficient translation symmetry. We find, that at the crossover the thermal energy is of the order of the energy of the strongest normal mode of the system and quantify the degree of entanglement below the critical temperature. Finally, we discuss the example of a ring topology in detail and compare our results with previous work in an entanglement-phase diagram.
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• Quantum Correlation Without Classical Correlations?
Dagomir Kaszlikowski, Aditi Sen (De), Ujjwal Sen, Vlatko Vedral, Andreas Winter
14 May 2007
Phys. Rev. Lett. 101, 070502 (2008)

Abstract:
Quantum and classical correlations play an important role in the sciences, stretching from quantum computation to DNA replication. We show that genuine multipartite quantum correlations can exist for states which have no genuine multipartite classical correlations, even in macroscopic systems. We construct such states for an arbitrary odd number of qubits. Such possibilities can have important implications in the physics of quantum information and phase transitions.
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• On maximal entanglement between two pairs in four-qubit pure states
Stephen Brierley, Atsushi Higuchi
23 April 2007
J. Phys. A: Math. Theor. 40 8455-8465 (2007)

Abstract:
We show that the state with the highest known average two-particle von Neumann entanglement entropy proposed by Sudbery and one of the authors gives a local maximum of this entropy. We also show that this is not the case for an alternative highly entangled state proposed by Brown et al.
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• Quantum search of partially ordered sets
Ashley Montanaro
21 February 2007
Quantum Information & Computation, vol. 9, no. 7&8, pp. 0628-0647, 2009

Abstract:
We investigate the generalisation of quantum search of unstructured and totally ordered sets to search of partially ordered sets (posets). Two models for poset search are considered. In both models, we show that quantum algorithms can achieve at most a quadratic improvement in query complexity over classical algorithms, up to logarithmic factors; we also give quantum algorithms that almost achieve this optimal reduction in complexity. In one model, we give an improved quantum algorithm for searching forest-like posets; in the other, we give an optimal O(sqrt(m))-query quantum algorithm for searching posets derived from m*m arrays sorted by rows and columns. This leads to an optimal O(sqrt(n))-query quantum algorithm for finding the intersection of two sorted lists of n integers.
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• Quantum coding theorem from privacy and distinguishability
Michal Horodecki, Seth Lloyd, Ansreas Winter
02 February 2007
Open Syst. Inf. Dyn. 15 (2008) 47-69

Abstract:
We prove direct quantum coding theorem for random quantum codes. The problem is separated into two parts: proof of distinguishability of codewords by receiver, and that of indistinguishability of codewords by environment (privacy). For a large class of codes, only privacy has to be checked.
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• A decoupling approach to the quantum capacity
Patrick Hayden, Michal Horodecki, Jon Yard, Andreas Winter
02 February 2007
Open Syst. Inf. Dyn. 15 (2008) 7-19

Abstract:
We give a proof that the coherent information is an achievable rate for the transmission of quantum information through a noisy quantum channel. Our method is to select coding subspaces according to the unitarily invariant measure and then show that provided those subspaces are sufficiently small, any data contained within them will with high probability be decoupled from the noisy channel's environment.
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• SECOQC White Paper on Quantum Key Distribution and Cryptography
Romain Alleaume, Jan Bouda, Cyril Branciard, Thierry Debuisschert, Mehrdad Dianati, Nicolas Gisin, Mark Godfrey, Philippe Grangier, Thomas Langer, Anthony Leverrier, Norbert Lutkenhaus, Philippe Painchault, Momtchil Peev, Andreas Poppe, Thomas Pornin, John Rarity, Renato Renner, Gregoire Ribordy, Michel Riguidel, Louis Salvail, Andrew Shields, Harald Weinfurter, Anton Zeilinger
24 January 2007

Abstract:
The SECOQC White Paper on Quantum Key Distribution and Cryptography is the outcome on a thorough consultation and discussion among the participants of the European project SECOQC (www.secoqc.net). This paper is a review article that attempts to position Quantum Key Distribution (QKD) in terms of cryptographic applications. A detailed comparison of QKD with the solutions currently in use to solve the key distribution problem, based on classical cryptography, is provided. We also detail how the work on QKD networks lead within SECOQC will allow the deployment of long-distance secure communication infrastructures based on quantum cryptography. The purpose of the White Paper is finally to promote closer collaboration between classical and quantum cryptographers. We believe that very fruitful research, involving both communities, could emerge in the future years and try to sketch what may be the next challenges in this direction.
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• Quantum interference with photon pairs using two micro-structured fibres
J. Fulconis, O. Alibart, W. J. Wadsworth, J. G. Rarity
19 January 2007
New J. Phys. 9 No 8 (August 2007) 276

Abstract:
We demonstrate a quantum interference experiment between two photons coming from non-degenerate pairs created by four-wave mixing in two separated micro-structured fibres. When the two heralded photons are made indistinguishable a 95% visibility is demonstrated.
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• Generalised Clifford groups and simulation of associated quantum circuits
Sean Clark, Richard Jozsa, Noah Linden
17 January 2007
Quant. Inform. Comp. vol 8, pp 106-126.(2008)

Abstract:
Quantum computations that involve only Clifford operations are classically simulable despite the fact that they generate highly entangled states; this is the content of the Gottesman-Knill theorem. Here we isolate the ingredients of the theorem and provide generalisations of some of them with the aim of identifying new classes of simulable quantum computations. In the usual construction, Clifford operations arise as projective normalisers of the first and second tensor powers of the Pauli group. We consider replacing the Pauli group by an arbitrary finite subgroup G of U(d). In particular we seek G such that G tensor G has an entangling normaliser. Via a generalisation of the Gottesman-Knill theorem the resulting normalisers lead to classes of quantum circuits that can be classically efficiently simulated. For the qubit case d=2 we exhaustively treat all finite subgroups of U(2) and find that the only ones (up to unitary equivalence and trivial phase extensions) with entangling normalisers are the groups G_n generated by X and the n^th root of Z.
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• A simple family of nonadditive quantum codes
John A. Smolin, Graeme Smith, Stephanie Wehner
15 January 2007
Phys. Rev. Lett. 99, 130505 (2007)

Abstract:
Most known quantum codes are additive, meaning the codespace can be described as the simultaneous eigenspace of an abelian subgroup of the Pauli group. While in some scenarios such codes are strictly suboptimal, very little is understood about how to construct nonadditive codes with good performance. Here we present a family of nonadditive quantum codes for all odd blocklengths, n, that has a particularly simple form. Our codes correct single qubit erasures while encoding a higher dimensional space than is possible with an additive code or, for n of 11 or greater, any previous codes.
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• Engineering correlation and entanglement dynamics in spin systems
T. S. Cubitt, J.I. Cirac
11 January 2007
Phys. Rev. Lett. 100:180406 (2008)

Abstract:
We show that the correlation and entanglement dynamics of spin systems can be understood in terms of propagation of spin waves. This gives a simple, physical explanation of the behaviour seen in a number of recent works, in which a localised, low-energy excitation is created and allowed to evolve. But it also extends to the scenario of translationally invariant systems in states far from equilibrium, which require less local control to prepare. Spin-wave evolution is completely determined by the system's dispersion relation, and the latter typically depends on a small number of external, physical parameters. Therefore, this new insight into correlation dynamics opens up the possibility not only of predicting but also of controlling the propagation velocity and dispersion rate, by manipulating these parameters. We demonstrate this analytically in a simple, example system.
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• Engineering correlation and entanglement dynamics in spin systems
T. S. Cubitt, J.I. Cirac
11 January 2007
Phys. Rev. Lett. 100:180406 (2008)

Abstract:
We show that the correlation and entanglement dynamics of spin systems can be understood in terms of propagation of spin waves. This gives a simple, physical explanation of the behaviour seen in a number of recent works, in which a localised, low-energy excitation is created and allowed to evolve. But it also extends to the scenario of translationally invariant systems in states far from equilibrium, which require less local control to prepare. Spin-wave evolution is completely determined by the system's dispersion relation, and the latter typically depends on a small number of external, physical parameters. Therefore, this new insight into correlation dynamics opens up the possibility not only of predicting but also of controlling the propagation velocity and dispersion rate, by manipulating these parameters. We demonstrate this analytically in a simple, example system.
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