Quantum Computation & Information Group seminars are listed below. The Theoretical Physics seminar series in the Physics department may also be of interest. Our seminars are announced by email; if you would like to be added to the mailing list, please contact Ashley Montanaro.

There are no upcoming seminars.

• 18 March 2010 16:00 (NSQI quantum information discussion room)
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• Simone Severini
• Patterns of zeros in matrices
• Host: Richard Jozsa


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Abstract:
Matrices have a double life: algebraic and combinatorial. The pattern formed by the positions of the zeros is an aspect of their combinatorial life. Despite I am not a matrix, it is also an aspect of my life, since the topic ended up to be part of my PhD research -- here, in the Alma Mater Bristol. This talk is a medley of young notions and results about patterns of zeros in matrices. In particular, I will consider orthogonal matrices and semidefinite rank problems, and I will discuss combinatorial parameters like the Colin de Verdiere and the Lovasz number. Some of the ideas linked with quantum information will be highlighted.
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• 10 March 2010 16:00 (NSQI quantum information seminar room)
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• Nilanjana Datta
• Relative entropies and entanglement monotones
• Host: Toby Cubitt


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Abstract:
We introduce two relative entropy quantities called the min- and max-relative entropies and discuss their properties and operational meanings. These relative entropies act as parent quantities for optimal one-shot rates of information-processing tasks such as data compression, information transmission and entanglement manipulation. Moreover, they lead us to define entanglement monotones which have interesting operational interpretations.
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• 03 March 2010 16:00 (NSQI quantum information seminar room)
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• Koji Azuma
• Optimal entanglement generation in hybrid quantum repeaters
• Host: Richard Jozsa


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Abstract:
We propose a realistic protocol to generate entanglement between quantum memories at neighboring nodes in hybrid quantum repeaters. Generated entanglement includes only one type of error, which enables efficient entanglement distillation and entanglement swapping. In contrast to the known protocols with such a property, our protocol with ideal detectors achieves the theoretical limit of the success probability and the fidelity to a Bell state, promising higher efficiencies in the repeaters. We also show that the advantage of our protocol remains even with realistic threshold detectors. This talk will be based on two papers: [1] Phys. Rev. A *80*, 060303(R) (2009); [2] arXiv: 0908.2735 (to be published in Phys. Rev. A).
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• 24 February 2010 16:00 (NSQI quantum information seminar room)
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• Terry Rudolph
• Entanglement and the thermodynamic arrow of time
• Host: Toby Cubitt


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Abstract:
Reference: D. Jennings and T. Rudolph, http://arxiv.org/abs/1002.0314 This talk will be about the types of "weird" processes that can occur - in particular heat flow from hotter to colder bodies - when systems and reservoirs (either classical or quantum) are correlated, contrary to the standard thermodynamic assumption of no initial correlations (i.e "molecular chaos"). It was motivated by Partovi's example of such a violation of the thermodynamic arrow, presented in [1] that he claimed was a manifestation of entanglement. It turns out that Partovi's example requires only classical correlation between the system and reservoir, and our motivation was to find out the difference between the case of initial quantum versus classical correlations. We show how to use the mutual information as a way of quantifying the amount of "negative heat flow", and show that the amount of such a flow can be larger for the case of initial entanglement, in some sense providing an entanglement witness different to those considered previously. I will also discuss Maccone's proposed "quantum resolution to the arrow of time"[2], and explain why it is flawed [3]. I regret this talk will not be as crazy as one would hope for from a talk with such a title. An indication of this is I will briefly mention some simple experiments that could be performed on small numbers of qubits to illustrate the main ideas. [1] H. Partovi, Phys. Rev. E 77, 021110 (2008), [2] L. Maccone, Phys. Rev. Lett. 103 080401 (2009) [3] D. Jennings and T. Rudolph, arxiv.org/abs/0909.1726
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• 16 February 2010 12:00 (NSQI quantum information seminar room / TUESDAY)
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• David Perez-Garcia
• A quantum version of Wielandt's inequality
• Host: Toby Cubitt


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Abstract:
We extend Wielandt's inequality to quantum channels. That is, we find upper bounds to the number of times a channel must be applied so that it maps any density operator to one with full rank. Using this bound we derive dichotomy theorems for the zero-error capacity of quantum channels and for the Matrix Product State (MPS) dimension of ground states of frustration free Hamiltonians. The obtained inequalities also imply new bounds on the required interaction-range of Hamiltonians with unique MPS ground state.
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• 10 February 2010 16:00 (NSQI QI seminar room)
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• Dieter Jaksch
• Entanglement Percolation with Bipartite Mixed States
• Host: Nicolas Brunner


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Abstract:
I will develop a concept of entanglement percolation for long-distance singlet generation in quantum networks where neighbouring nodes are connected by partially entangled bipartite mixed states. I will give a necessary and sufficient condition on the class of mixed network states for the generation of singlets. I will discuss that neighbouring nodes are required to be connected by multiple partially entangled states to achieve entanglement percolation. Furthermore I will devise a rich variety of distillation protocols for the conversion of these mixed states into singlets. Finally, I will introduce possible further improvements achievable by using quantum strategies including generalized forms of entanglement swapping.
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• 27 January 2010 16:00 (NSQI quantum information seminar room)
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• Jiannis Pachos
• The drunken slalom
• Host: Toby Cubitt


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Abstract:
After a short introduction on anyons and quantum walks we shall present the one dimensional quantum walk of anyonic systems. The anyonic walker performs braiding operations with stationary anyons of the same type ordered canonically on the line of the walk. Abelian as well as non-Abelian anyons are considered that exhibit dramatically different behaviours. The relation between walker position and Jones polynomials, topological invariants of links and knots, is established.
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• 13 January 2010 16:00 (NSQI quantum information seminar room)
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• Vlad Gheorghiu
• Location of Quantum Information in Additive Graph Codes
• Host: Richard Jozsa


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Abstract:
Unlike classical information, quantum information can be present in more than one "type", and various types can be incompatible (e.g. the information represented by the X component of a spin one-half particle versus its Z component counterpart). In this talk I will introduce the notion of "type of information" and I will demonstrate how to use this concept to study the location of quantum information in arbitrary carrier subsets of additive graph codes. To various types of information we associate a collection of operators on the coding space which form what we call the information group. It represents the input information through an encoding operation constructed as an explicit quantum circuit. Partial traces of these operators down to a particular subset of carriers provide an isomorphism of a subgroup of the information group, and this gives a precise characterization of what kinds of information they contain. All carriers are assumed to have the same dimension D, an arbitrary integer greater than 1 (not necessarily prime). The talk will be pedagogical and will be accompanied by a series of illustrative examples. No special pre-requisites are required. For a detailed discussion the interested reader can consult our work at arXiv:0912.2017 [quant-ph].
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• 17 December 2009 16:00 (NSQI quantum information seminar room)
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• Katherine Brown (Leeds)
• An optimally efficient technique for the generation of cluster states using a qubus quantum computer.
• Host: Clare Horsman


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Abstract:
The measurement based or cluster state quantum computer is a scheme which is universal for quantum computing. Measurments are performed on a highly entangled 2 or 3 dimensional lattice, with the result of one measurment determining the basis for the next. In such a scheme the generation of entanglement is pushed offline which means any errors in this process can be corrected before calculations take place. In this talk I will introduce a deterministic method of generating cluster states using a qubus architecture. This architecture uses a continuous variable field to generate entanglement between the qubits, in this case the C-Phase gates required to generate a cluster state. The method I present for generating cluster states is dynamic allowing the cluster state to be grown in one dimension to any desired size. I show that my method shows a dramatic improvement over a naive method of cluster state generation using the qubus and an improvement over previous work also showing an improvement.
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• 16 December 2009 16:00 (NSQI quantum information seminar room)
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• Toby Cubitt
• Super-duper-activation of the Zero-Error Quantum Capacity


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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. The study of zero-error capacities dates right back to Shannon and the early days of information theory. 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. We then strengthen this result to show that there exist pairs of channels, neither of which have any zero-error classical capacity (as before), yet for which access to a single copy of the joint channel even allows far more delicate quantum information to be transmitted perfectly. This subsumes the first result, and it also implies that the quantum zero-error capacity can be superactivated. Indeed, these channels exhibit superactivation of both their classical and quantum zero-error capacities simultaneously. This is the strongest conceivable form of superactivation, and nothing similar is possible for standard Shannon capacities of quantum channels or for zero-error capacities of classical channels. We term this striking effect "super-duper-activation".
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• 09 December 2009 14:15 (Physics / 3.27)
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• Renato M. Angelo (UFPR - Brazil / Bristol)
• The paradoxical nature of quantum references frames


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Abstract:
Consider a quantum system composed of N+1 particles. How does one of the particles of the system (now promoted to the role of a "quantum observer") describe the joint state of the other N particles? In this work we investigate fundamental questions emerging in this scenario. We uncover a paradox in which an arbitrarily distant particle is found to dramatically influence the physics in the vicinity of the quantum observer. In solving the puzzle some crucial features of quantum reference frames are identified.
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• 03 December 2009 16:00 (NSQI quantum information seminar room / THURSDAY)
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• Will Matthews
• Improving zero-error classical communication with entanglement
• Host: Ashley Montanaro


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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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• 02 December 2009 16:00 (NSQI QI seminar room)
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• Janet Anders
• Quantum correlations and complexity
• Host: Andreas Winter


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Abstract:
Quantum particles have the ability to be very strongly correlated, a phenomenon called entanglement. In my talk I will explain how the presence of quantum correlations can assist in performing "impossible" computational tasks [1]. I will then show how quantum correlations can be used to "remote control" any quantum system, with the help of a single auxiliary qubit [2]. While these are man made, externally controlled applications of quantum correlations one may wonder if existing natural processes exploit their power. I will show how quantum correlations can help "squeeze" energy in many physical situations by forcing collective behaviour. Finally, I would like to debate with you, if a single massive Boson that occupies a confined volume results in spatial regions of the volume to be entangled and how this may be tested with a Bell-inequality [3]. References: [1] J. Anders, D. E. Browne, Phys Rev Lett 102 050502 (2009). [2] J. Anders, D.K.L. Oi, E. Kashefi, D.E. Browne, E. Andersson, arXiv:0911.3783v1 (2009). [3] L. Heaney, J. Anders, Phys Rev A 80 032104 (2009).
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• 24 November 2009 16:00 (NSQI QI seminar room / TUESDAY)
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• Almut Beige
• Something from nothing: Energy concentration in atom-cavity systems
• Host: Nicolas Brunner


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Abstract:
The spontaneous emission of photons from optical cavities and from individually trapped atoms has been studied extensively in the framework of quantum optics. Theoretical predictions based on the rotating wave approximation (RWA) are in general in very good agreement with experimental findings. However, current experiments aim at combining better and better cavities with large numbers of tightly confined atoms. Here we predict an energy concentrating mechanism in the behavior of such a composite quantum system which cannot be described by the RWA. Its result is the continuous leakage of photon through the cavity mirrors even in the absence of external driving and even when there are initially no photons inside the resonator. We finally discuss the predicted phenomenon in the context of thermodynamics.
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• 18 November 2009 16:00 (NSQI quantum information seminar room)
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• Alastair Kay
• The Entanglement Phases of Thermal Graph States
• Host: Nicolas Brunner


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In order to categorise and quantify the types and amount of entanglement that we might find in real physical systems (described by local Hamiltonians), we discuss these issues for the thermal states of graphs. After a brief review of the distillability properties of these graphs, we calculate the critical temperatures for the persistence k-partite bound entanglement phases present in this model for a restricted class of graphs including, for example, the 1D cluster state.
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• 05 November 2009 16:00 (NSQI quantum information seminar room)
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• Matthias Christandl
• The Uncertainty Principle in the Presence of Quantum Memory
• Host: Richard Jozsa


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Abstract:
Quantum mechanical uncertainty relations provide bounds on the minimum uncertainties about the outcomes of two alternative measurements applied to the same quantum state. In this paper, we prove an entropic uncertainty relation which, in contrast to known such relations, is valid in the context of quantum side information. It strengthens and extends the entropic uncertainty relation of Maassen and Uffink [Phys. Rev. Lett. 60, 1103 (1988)] and also implies an inequality recently conjectured by Boileau and Renes [Phys. Rev. Lett. 103, 020402 (2009)].The proof uses the formalism of smooth quantum entropies.
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• 28 October 2009 16:00 (NSQI quantum seminar room)
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• Jonathan Allcock
• Closed sets of non-local correlations


• 07 October 2009 16:00 (NSQI seminar room)
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• Frederic Dupuis (Universite de Montreal)
• The decoupling approach to quantum information theory revisited


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Abstract:
Many important theorems in quantum information theory can be proven in a particularly simple manner by showing that two quantum systems are "decoupled" (meaning that they are in a product state). In this talk, I will present a theorem that generalizes most of the methods that are used to show that two systems are decoupled. This yields simplified and more direct proofs of the correctness of many protocols.
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• 01 October 2009 16:00 (NSQI seminar room)
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• Stephanie Wehner (Caltech)
• Unconditional security from noisy quantum storage


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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. Joint work with Robert Koenig and Juerg Wullschleger
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• 09 September 2009 16:00 (NSQI Building seminar room)
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• Tim Ralph (University of Queensland)
• Can Gravitational Curvature Decorrelate Entanglement?
• The NSQI Building is on Tyndall Avenue, next to the Physics department


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Abstract:
I will discuss an alternative formulation of the problem of quantum optical fields in a curved space-time inspired by quantum mechanical consistency requirements in exotic metrics. I will contrast this new formulation with the standard approach and find observable differences for entangled states. I will propose an experiment in which an entangled pair of optical pulses are propagated through non-uniform gravitational fields and find that the new formulation predicts de-correlation of the optical entanglement under experimentally realistic conditions.
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• 09 September 2009 14:00 (NSQI seminar room)
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• Harry Buhrman
• A generalized Grothendieck inequality and entanglement in XOR games
• http://arxiv.org/abs/0901.2009


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Suppose Alice and Bob make local two-outcome measurements on a shared entangled state. For any d, we show that there are correlations that can only be reproduced if the local dimension is at least d. This resolves a conjecture of Brunner et al. and establishes that the amount of entanglement required to maximally violate a Bell inequality must depend on the number of measurement settings, not just the number of measurement outcomes. We prove this result by establishing the first lower bounds on a new generalization of Grothendieck's constant. Joint work with Jop Briet and Ben Toner
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• 17 June 2009 16:00 (3.34 Physics Department)
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• Yutaka Shikano (Tokyo)
• Weak Values with Decoherence


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Abstract:
What is probability in Quantum Mechanics? Considering this conceptual problem, a weak value may be a useful tool to discuss the mathematical property of the probability space. Furthermore, the weak value of an observable is experimentally accessible by weak measurements as theoretically analyzed by Aharonov et al. and recently experimentally demonstrated. We introduce the weak operator associated with the weak value and give a general framework of quantum operations to the weak operator in parallel with the Kraus representation of the completely positive map for the density operator. The decoherence effect is also investigated in terms of the weak measurement by a shift of a probe wave function of continuous variable [1]. In this talk, we introduce the weak value from the view of the probability space and a new technique to get the weak values from the analogy of the quantum eraser [2]. Finally we discuss the general framework of the weak values with decoherence. [1] Y. Shikano and A. Hosoya, arXiv:0812.4502. [2] S. Tamate, H. Kobayashi, T. Nakanishi, K. Sugiyama, and M. Kitano, arXiv:0906.0212.
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• 10 June 2009 16:00 (3.21 Physics Department)
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• Miguel Navascues (Imperial)
• A complete criterion for separability detection


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Using new results on the separability properties of bosonic systems, we provide a new complete criterion for separability. This criterion aims at characterizing the set of separable states from the inside by means of a sequence of efficiently solvable semidefinite programs. We apply this method to derive arbitrarily good approximations to the optimal measure-and-prepare strategy in generic state estimation problems. Finally, we report its performance in combination with the criterion developed by Doherty et al. [1] for the calculation of the entanglement robustness of a relevant family of quantum states whose separability properties were unknown. [1] A. C. Doherty, P. A. Parrilo, F. M. Spedalieri, Phys. Rev. Lett. 88, 187904 (2002).
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• 05 June 2009 14:00 (3.21 Physics Department)
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• Andreas Winter
• Information causality and all that


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Abstract:
By popular demand I'm going to present the details of the paper arXiv:0905.2292. With respect to a nonlocal game based on the idea of random access codes, I will show that quantum theory obeys a certain information inequality - which we dubbed "information causality" in the paper. Assuming this relation, one can then derive Tsirelson's bound on the CHSH correlator, and also recover other bounds on the quantum region within the non-signalling polytope. The question arises whether by adopting information causalty as a fundamental (intuitive?) principle, one can recover the predictions of quantum theory in all non-signalling contexts.
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• 01 June 2009 16:00 (3.34 Physics Department)
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• Andrzej Dragan (University of Warsaw)
• Emergence of quantum indeterminacy from special relativity


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Single-particle quantum theory with fundamental indeterminacy based on complex probability amplitudes undergoing linear superposition principle is an inevitable consequence of special relativity with the principle of relativity extended to superluminal observers.
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• 27 May 2009 16:00 (3.21 Physics Department)
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• Animesh Datta (Imperial)
• The Role of Entanglement in Quantum Computation - A Discord/ant/ story


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Abstract:
We attempt at characterizing the correlations present in the quantum computational model DQC1, introduced by Knill and Laflamme [Phys. Rev. Lett. 81, 5672 (1998)]. The model involves a collection of qubits in the completely mixed state coupled to a single control qubit that has nonzero purity. Although there is little or no entanglement between two parts of this system, it provides an exponential speedup in certain problems. On the contrary, we find that the quantum discord across the most natural split is nonzero for typical instances of the DQC1 ciruit. Nonzero values of discord indicate the presence of nonclassical correlations. We propose quantum discord as figure of merit for characterizing the resources present in this computational model. This might be a complementary measure for counting resources in quantum information science.
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• 20 May 2009 14:15 (3.27 Physics Department)
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• Tobias Osborne (Royal Holloway)
• Solving frustration-free hamiltonians
• Theoretical Physics seminar series


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Abstract:
Frustration-free hamiltonians, i.e. hamiltonians whose ground state is locally a ground state for each interaction term, arise in a variety of applications in condensed matter physics and quantum information theory. Examples include the AKLT model and Rokhsar-Kivelson models in condensed matter physics and quantum satisfiability in quantum information theory. Due to the absence of frustration in these systems it is tempting to conjecture that all frustration-free systems can also be solved locally and efficiently (i.e., using only a polynomial number of arithmetic operations in the number of quantum spins). In this talk I'll show for a large class of such systems that this is indeed the case. I'll also review a class of frustration-free systems in 1D which provably cannot be solved efficiently.
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• 13 May 2009 16:00 (3.34 Physics Department)
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• Richard Low
• Large Deviation Bounds for k-designs


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Random unitaries are very useful things with many applications. There are many results that say a unitary picked at random will have desirable properties with high probability. However, these results are not very physically or computationally relevant because random unitaries cannot be constructed efficiently. In this talk, I take a pseudo-random set of unitaries (called a k-design) that can be efficiently constructed and show that in many cases the desirable properties of truly random unitaries can be reproduced. I will then illustrate this with examples, including showing that pseudo-random unitaries are enough to give the 'principle of equal a priori probabilities' from statistical mechanics, in the sense of Popescu, Short & Winter, Nature Physics (2006).
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• 06 May 2009 16:00 (3.21 Physics Department)
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• Valerio Scarani (University of Singapore)
• QKD: a million signal task


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Would you believe that all the claims of "unconditional security" made prior to the year 2007, both by theorists and experimentalists, were valid only under the assumption that the secret key is of *infinite length*? A few people were actually aware of the problem, but the mathematical complexity of the task seemed beyond reach. Then came Renato Renner, who invented all the tools in his thesis. It just needed someone to dig out the meaningful formulas, put them in Matlab and compute numbers --- and this is the kind of task even the present speaker can contribute to :-) The result was shocking for many: you cannot extract any secret key at all unless you process 10^5 or 10^6 signals (you guess it: very few experiments, if any, complied). The asymptotic bounds are recovered only when the number of signals goes beyond 10^10, which is simply beyond reach. In this short talk, I shall show how the finite-key security bound looks ultimately very natural, and present in detail some of the results for the BB84 protocol in various implementations.
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• 23 April 2009 16:00 (3.21 Physics Department)
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• Marco Piani (IQC, Waterloo)
• Relative Entropy of Entanglement and Restricted Measurements


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We introduce variants of relative entropy of entanglement based on the optimal distinguishability from unentangled states by means of restricted measurements. In this way, we are able to prove that the standard regularized entropy of entanglement is strictly positive for all multipartite entangled states. In particular, this implies that the asymptotic creation of a multipartite entangled state by means of local operations and classical communication always require the consumption of a non-local resource at a strictly positive rate.
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• 22 April 2009 16:00 (3.21 Physics Department)
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• Fernando Brandao (Imperial)
• The complexity of poly-gapped Hamiltonians: Extending Valiant-Vazirani theorem to the probabilistic and quantum settings


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Valiant-Vazirani showed in 1985 that solving NP with the promise that "yes" instances have only one witness is powerful enough to solve the entire NP class (under randomized reductions). We are interested in extending this result to the quantum setting. We prove extensions to the classes Merlin-Arthur (MA) and Quantum-Classical-Merlin-Arthur (QCMA). Our results have implications on the complexity of approximating the ground state energy of a quantum local Hamiltonian with a unique ground state and an inverse polynomial spectral gap. We show that the estimation, to within polynomial accuracy, of the ground state energy of poly-gapped 1-D local Hamiltonians is QCMA-hard, under randomized reductions. This is in strong contrast to the case of constant gapped 1-D Hamiltonians, which is in NP. Moreover, it shows that unless QCMA can be reduced to NP by randomized reductions, there is no classical description of the ground state of every poly-gapped local Hamiltonian which allows the calculation of expectation values efficiently. Finally, we discuss a few obstacles towards establishing an analogous result to the class Quantum-Merlin-Arthur (QMA). In particular, we show that there is no black box reduction, micking the construction of valiant and Vazirani, in the quantum case. This is joint work with Dorit Aharonov, Michael Ben-Or and Or Sattath.
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• 25 March 2009 16:00 (3.21 Physics Department)
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• Eric Chitambar (University of Michigan)
• Encoding Various Decision Problems as a Question of Entanglement Convertibility
• Note change of room


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Abstract:
Many computational problems can be recast in terms of whether some initial entangled state can be transformed into another by local operations and classical communication. While the problems may appear completely unrelated, they share a common mathematical structure and thus the same set of necessary and sufficient conditions. In this talk, I will discuss a few examples of problems that can be solved by analyzing bipartite/tripartite entanglement conversions and vice versa. These examples include the bipartite matching problem and the calculation of tensor rank.
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• 18 March 2009 16:00 (3.34 Physics Department)
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• Sarah Leyton (Royal Holloway)
• A Quantum Algorithm to solve Nonlinear Differential Equations


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Abstract:
I will present a quantum algorithm to solve sparse systems of nonlinear differential equations. The algorithm is nondeterministic and its expected resource requirements are poly-logarithmic in the number of variables and exponential in the integration time. The best classical algorithm runs in a time scaling linearly with the number of variables, so this provides an exponential improvement.
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• 11 March 2009 16:00 (3.21 Physics Department)
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• Daniel Burgarth (Imperial)
• Indirect Hamiltonian Estimation and Restricted Control
• Note change of room.


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Abstract:
I will discuss the indirect estimation of Hamiltonian parameters through measuring a few qubits of a large coupled graph. This can be seen as an instance of system identification. Then I will speak about the controllability of the graph through restricted operations on a few qubits. The two problems have a joint graph-theoretical criterion.
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• 25 February 2009 16:00 (3.34 Physics Department)
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• Sandu Popescu
• Entanglement in biological systems


• 18 February 2009 16:00 (3.34 Physics Department)
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• Joe Fitzsimons (Oxford)
• Communication and Blind Quantum Computation


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Abstract:
In this talk I will introduce the notion of 'blind quantum computation'. This term refers to the problem of allowing Alice to have Bob carry out a quantum computation for her such that Bob learns nothing about the computation that he performs. I will briefly review a protocol for accomplishing this task securely, which we presented in arXiv:0807.4154, before discussing the communications complexity of blind quantum computing in general. A generalisation of our original protocol can be shown to have optimal scaling of the required quantum communication.
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• 04 February 2009 16:00 (3.34 Physics Department)
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• Paul Skrzypczyk
• Non-locality swapping


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Abstract:
I will talk about implementing the analogue of entanglement swapping in a generalised probabilistic setting (PR boxes). This will require a new concept, that of genuine boxes. I will motivate this and show how it leads to a working non-locality swapping protocol. Finally, I will discuss a surprising, and still mysterious, connection to quantum mechanics.
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• 28 January 2009 16:00 (3.34 Physics Department)
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• Aram Harrow
• Quantum algorithm for solving linear systems of equations


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Abstract:
Given a matrix A and a vector b as input, I will describe a quantum algorithm for finding a vector x satisfying Ax=b. The answer is given not as a list of numbers, but as a quantum state |x> that is proportional to the vector x. The run-time scales with the log of the dimension of A, as well as polynomially with the desired accuracy, the sparsity of A (number of nonzero entries per row) and its condition number (ratio between largest and smallest singular values). I will also review classical algorithms for this problem and give a proof that, relative to an oracle, such algorithms cannot be substantially improved upon. Thus, for suitable matrices, our algorithm runs exponentially faster than any possible classical algorithm. This is joint work with Avinatan Hassidim and Seth Lloyd. The arxiv version (0811.3071v1) is somewhat out of date. If you want to read it, I suggest either downloading the paper from this link http://www.maths.bris.ac.uk/~csawh/misc/inversion.pdf or waiting until 0811.3071v2.
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• 07 January 2009 16:00 (3.34 Physics Department)
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• Ivette Fuentes-Schuller (Technical University of Berlin)
• Entanglement in non-inertial frames and curved spacetime


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Abstract:
The insight that the world is fundamentally quantum mechanical inspired the development of quantum information theory. However, the world is not only quantum but also relativistic, and indeed many implementations of quantum information tasks involve truly relativistic systems. In this talk I consider relativistic effects on entanglement in flat and curved spacetimes. I will emphasize the qualitative differences to a non-relativistic treatment, and demonstrate that a thorough understanding of quantum information theory requires taking relativity into account. The exploitation of such relativistic effects will likely play an increasing role in the future development of quantum information theory. The relevance of these results extends beyond pure quantum information theory, and applications to foundational questions in cosmology and black hole physics will be presented.
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• 17 December 2008 16:00 (3.34 Physics Department)
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• Debbie Leung (IQC, Waterloo)
• Continuity of quantum channel capacities


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Abstract:
Many capacities of a quantum channel are given by expressions that involve asymptotically large number of uses of the channel, thus, there is no a priori reason for the capacities to be continuous. In this talk, we prove that the ability to transmit classical data, private classical data, quantum data with or without free classical communication on the side are all continuous. Joint work with Graeme Smith from IBM TJ Watson Research Center.
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• 15 December 2008 16:00 (3.34 Physics Department)
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• Sarah Croke (Perimeter Institute)
• Approximating quantum operations using weak values


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Abstract:
When a pre and post-selected quantum system interacts weakly with a probe system, the so-called weak value of an observable of the system is imprinted onto the probe. The weak value can take values outside the range of the eigenvalues of the associated observable, and can even be complex. This effect has been studied extensively to investigate the properties of quantum systems in between measurements, in the time symmetric or two-state vector formalism, and has proven to be a fruitful concept in studying foundational issues in quantum mechanics. Taking a more operational approach, the imprinting of weak values can be exploited to perform useful tasks in quantum information theory, such as in quantum communication schemes, in entanglement concentration, and to amplify small optical effects, allowing them to be measured. Nevertheless, this potentially useful effect has remained relatively unexplored. In this talk I will discuss the use of the weak measurement formalism to perform approximate transformations on the probe system, including transformations which cannot be performed by any physically allowed generalised measurement. In particular the weak measurement formalism provides a conceptually simple scheme to probabilistically realise noiseless amplification of quantum optical states. I will discuss some applications of this effect, including continuous variable entanglement concentration and high-fidelity cloning of small amplitude coherent states. Joint work with David Menzies of the University of St Andrews.
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• 10 December 2008 16:00 (3.34 Physics Department)
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• Michael Bremner
• Instantaneous Quantum Computation


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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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• 04 December 2008 16:00 (3.34 Physics Department)
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• Elham Kashefi (University of Edinburgh)
• Computing without Trusting


(view abstract)
Abstract:
We present a protocol which allows a client to have a server carry out a quantum computation for her such that the client's inputs, outputs and computation remain perfectly private, and where she does not require any quantum computational power or memory. The client only needs to be able to prepare single qubits randomly chosen from a finite set and send them to the server, who has the balance of the required quantum computational resources. Our protocol is interactive: after the initial preparation of quantum states, the client and server use two-way classical communication which enables the client to drive the computation, giving single-qubit measurement instructions to the server, depending on previous measurement outcomes. Our protocol works for inputs and outputs that are either classical or quantum. We give an authentication protocol that allows the client to detect an interfering server; our scheme can also be made fault-tolerant. Our protocol is the first universal scheme which is independent of the properties of the function to be evaluated. This is in contrast with other classical or quantum approaches based on the notion of computing with encoded data, which exploit the properties of the underlying function to derive an efficient encoding and decoding algorithms. However the novelty of our approach is in the using of the unique feature of the measurement-based quantum computing that separates classical and quantum part of computation and will lead to a generic scheme for blind computation of any arbitrary function.
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• 19 November 2008 16:00 (SM3 Mathematics Department)
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• Ashley Montanaro
• Quantum boolean functions


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Abstract:
In recent years, the analysis of boolean functions has arisen as an important theme in theoretical computer science. In this talk I will discuss an extension of the concept of a boolean function to quantum computation. It turns out that many important classical results in the theory of boolean functions have natural quantum analogues. These include property testing of boolean functions; the Goldreich-Levin algorithm for approximately learning boolean functions; and a theorem of Friedgut, Kalai and Naor on the Fourier spectra of boolean functions. The quantum generalisation of this theorem uses a quantum extension of the hypercontractive inequality of Bonami, Gross and Beckner. This talk is based on joint work with Tobias Osborne.
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• 12 November 2008 16:00 (SM3 Mathematics Department)
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• Koenraad Audenaert (Royal Holloway)
• Quantum Tomographic Reconstruction with Error Bars: a Kalman Filter Approach


(view abstract)
Abstract:
We present a novel quantum tomographic reconstruction method based on Bayesian inference via the Kalman filter update equations. The method not only yields the maximum likelihood/optimal Bayesian reconstruction, but also a covariance matrix expressing the measurement uncertainties in a complete way. From this covariance matrix the error bars on any derived quantity can be easily calculated. This is a first step towards the broader goal of devising an omnibus reconstruction method that could be adapted to any tomographic setup with little effort and that treats measurement uncertainties in a statistically well-founded way. We restrict ourselves to the important subclass of tomography based on measurements with discrete outcomes (as opposed to continuous ones), and we also ignore any measurement imperfections (dark counts, less than unit detector efficiency, etc.), which will be treated in further work. We illustrate the general theory on two real tomography experiments of quantum optical information processing elements.
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• 29 October 2008 16:00 (SM3 Mathematics Department)
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• Gavin Brennen (Macquarie University)
• Nonlocality of Non-Abelions


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Abstract:
Non-local correlations are a central theme in quantum mechanics and tell us that there exist states in the joint Hilbert space of subsystems that have measurement outcomes not describable by local realism. There is another kind of non-locality arising from topological interactions amoung anyons which is metric independent. I will describe how the later concept can be understood in terms of the former via a Bell like inequality between measurements on the fusion products of the anyons.
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• 22 October 2008 16:00 (SM3 Mathematics Department)
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• Jonathan Matthews
• Integrated Photonics Quantum Circuits


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Abstract:
Recently, the field of linear quantum optics has witnessed for the first time, the successful demonstration of lithographically etched silica-on-silicon monolithic integrated waveguide structures for single photon-pair experiments including high visibility non-classical interference and high fidelity implementation of a linear optical C-NOT quantum logic gate [1]. The integrated nature of these devices allows experiments to be reliably and stably realized on sophisticated photonic quantum circuits that would otherwise be inherently difficult to implement in bulk optics. We report the next step for silica photonic circuits by demonstrating stable and accurate control of single qubits and maximally entangled fock states. These results have direct application in quantum metrology [2], as well as more generally to fundamental quantum optics and optical quantum information processing [3]. Waveguide circuits can also be directly written into materials such as fused silica using tightly-focused femto-second lasers [4]; this technique has since been used to inscribe integrated devices such as monolithic waveguide-DFB-lasers [5]. We report here our progress on applying this technology to experiments in quantum optics. [1] A Politi, M Cryan, J G Rarity, S Yu, J L O'Brien, Science 320, 646 (2008) [2] T Nagata, R Okamoto, J L O'Brien, K Sasaki, S Takeuchi, Science 316, 726 (2007) [3] J L O'Brien, Science 318 1567 (2007) [4] K. M. Davis, K. Miura, N. Sugimoto, K. Hirao, Optic Lett, 21, 21 (1996). [5] G. D. Marshall, P. Dekker, M. Ams, J. A. Piper, and M. J. Withford, Opt. Lett., 33, 33 (2008).
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• 15 October 2008 16:00 (SM3 Mathematics Department)
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• Aggie Branczyk (University of Queensland)
• Quantum Computing with Kittens


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Abstract:
In this talk I will give an introduction to cat states and coherent state quantum computing (CSQC) as well as briefly discuss how approximations to small-amplitude cat states (kitten states) can be realised using squeezed light. I will then present theoretical results of how well we could expect these approximate kitten states to work in proof-of-principle demonstrations of coherent state teleportation.
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• 08 October 2008 14:30 (SM3 Mathematics Department)
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• Michel Planat (Institut FEMTO-ST, CNRS)
• Reflection groups for quantum computing


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Abstract:
I explore the representation of quantum computing in terms of unitary reflection groups (groups of unitary transformations that leave invariant a hyperplane of a vector space). The Clifford group Cn normalizes the n-qubit Pauli group Pn (tensor products of Pauli matrices up to a phase) within the unitary group. Clifford group appeared in the context of self-dual error-correcting codes (E Bannai 99, G Nebe 00), and it is well known that C1 and C2 are related to complex reflection groups of the Shephard-Todd sequence. In quantum computing, Gottesman 97 discovered that Cn may be generated using two single qubit gates (Hadamard and phase Pi/4 gates) and a two-qubit gate (Controlled Not gate). In this work [Unitary reflection groups for quantum fault tolerance. Michel Planat and Maurice Kibler. Preprint 0807.3650 [quantph]. ], one uses some cristallographic reflection groups over the real Euclidean space (Coxeter groups), as well as imprimitive complex reflection groups. Imprimitive groups are related to the automorphisms of mutually unbiased bases of Pn. Coxeter groups of type B3 and G2 control the automorphisms of P1, and Coxeter systems of type D5 and E6 appear in the topological component of C2 and C3. There is thus a link with the geometry of smooth cubic surfaces. Finally group C3 contain the symmetries of octonions through its unique subgroups of order 168, 6048 and 12096, that are isomorphic to Lie groups PSL(2,7), SU(3,3) and G2(2).
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• 07 October 2008 16:00 (Physics 3.34)
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• Simon Devitt, National Institute for Informatics, Tokyo, Japan
• Topological optical Cluster state computation with the photonic module.


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Abstract:
Optical quantum computation has represented one of the most successful testbed systems for quantum information processing. Along with ion-traps and Nuclear Magnetic Resonance (NMR), experimentalists have demonstrated control of qubits, multi-qubit gates and small quantum algorithms. However, photonic based qubits suffer from a problematic lack of a large scale architecture for fault-tolerant computation which could conceivably be built in the near future. While optical systems are, in some regards, ideal for quantum computing due to their high mobility and low susceptibility to environmental decoherence, these same properties make the construction of compact, chip based architectures difficult. Here I discuss many of the important issues related to scalable fault-tolerant quantum computation and introduce a feasible architecture design for an optics based computer. We combine the recent development of topological cluster state computation with the photonic module, simple chip based devices which can be utilized to deterministically entangle photons. The integration of this operational unit with one of the most exciting computational models solves many of the existing problems with other optics based architectures and leads to a feasible large scale design which can continuously generate a 3D cluster state with a photonic module resource cost linear in the cross sectional size of the cluster.
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• 01 October 2008 16:00 (SM3 Mathematics Department)
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• Leandro Aolita (ICFO, Barcelona)
• Scaling laws for the decay of multiqubit entanglement


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Abstract:
We investigate the decay of entanglement of generalized $N$-particle Greenberger-Horne-Zeilinger (GHZ) states interacting with independent reservoirs. Scaling laws for the decay of entanglement and for its finite-time extinction (sudden death) are derived for different types of reservoirs. The latter is found to increase with the number of particles. However, entanglement becomes arbitrarily small, and therefore useless as a resource, much before it completely disappears, around a time which is inversely proportional to the number of particles. We also explore analytically and numerically how this behavior extends to other (more general) families of states. Finally, we show that the decay of multi-particle GHZ states can generate bound entangled states.
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• 20 August 2008 16:25 (3.34 Physics Department)
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• Ashley Montanaro
• Quantum algorithms for shifted subset problems


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Abstract:
One of the great successes of the field of quantum computation is the efficient quantum algorithm for the abelian hidden subgroup problem. In this talk, I'll discuss a generalisation of this task: 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. In particular, I'll 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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• 20 August 2008 16:00 (3.34 Physics Department)
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• Richard Low
• Random Quantum Circuits are Approximate 2-designs


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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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• 13 August 2008 16:00 (3.34 Physics Department)
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• Toby Cubitt (Bristol)
• On the complexity of deciding whether a quantum channel is Markovian


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Abstract:
As was leaked to the press a long time ago, I have shown that deciding Markovianity of a quantum channel is NP-hard. (The people responsible for this leak will shortly be shipped off to the darkest depths of Asia.) What has not yet been leaked is the details and proof of this result, which I will attempt to explain in this talk. A quantum channel is Markovian iff it is a member of a one-parameter completely positive semi-group, i.e. if it can be generated by a master equation. Why should you care about this seemingly esoteric problem in the theory of CP-maps? Because master equations are the (most general possible) dynamical equations of a quantum system; they describe the underlying physics that produces the evolution. Whereas quantum channels describe the (most complete possible) measurements of that evolution; they describe what we can actually observe in the laboratory. NP-hardness of the Markovianity problem implies that extracting the dynamical equations of a quantum system from any sequence of measurements would take longer than the lifetime of the universe for even moderately sized systems (assuming P /= NP). This is a profound limitation imposed by quantum mechanics itself, raising intriguing conceptual question about quantum mechanics and quantum experiments, and is in stark contrast to the analogous problem in classical physics (which is in P).
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• 06 August 2008 16:00 (3.34 Physics Department)
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• Howard Wiseman (Griffith University)
• Experimentally achieving Heisenberg-limited interferometry using Quantum Information, Measurement, and Control Theory


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Abstract:
The ability to manipulate individual quantum systems, using quantum measurement and control, promises to bring about a quantum information revolution, including computers that are faster than any conceivable conventional computer. A recent application of ideas from these disciplines is the first ever experiment achieving the Heisenberg limit in interferometry [Higgins et al., Nature 450, 393-6 (2007).] Interferometry is used ubiquitously, for example to measure the optical properties of a sample (e.g. its refractive index). The Heisenberg limit represents the most accurate possible measurement given a limited resource (e.g. the number of photon-passes through the sample). We demonstrated this using single-photon technology developed for quantum computing, implementing an algorithm inspired by quantum computer algorithms, and modified using measurement and control theory.
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• 02 July 2008 16:00 (3.34 Physics Department)
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• Sean Barrett (Imperial College)
• Transitions in universality for measurement-based quantum computation in a quantum many-body system


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Abstract:
The quantum state of a many-body system can serve as a resource for a method of quantum computing based solely on adaptive local measurements. We show that the usefulness of the thermal state of a specific spin-lattice model for measurement-based quantum computing exhibits a sharp transition between two distinct 'phases' -- one in which every state is a universal resource for quantum computation, and other in which any local measurement sequence can be simulated efficiently on a classical computer. Remarkably, this transition in computational power does not coincide with any conventional phase transition in the underlying model; in fact, we demonstrate that this model does not possess any phase transitions, classical or quantum. The methods used to obtain our central results will likely have other applications - in particular we have obtained a method for correcting global, correlated errors in cluster state preparation, while on the other hand our method for classically simulating thermal projected entangled pair states (PEPS) should be applicable to a broad class of systems.
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• 25 June 2008 16:00 (3.34 Physics Department)
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• John Mahoney and Karoline Wiesner
• An introduction into computational mechanics and the connection to quantum dynamical systems


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Abstract:
Computational mechanics is a mathematical framework for constructing a model of a physical process where the result is argued to be unique/physical/natural. We introduce the causal state equivalence class that is the basis for this physicality. Further, measures defined on this model will serve as well motivated complexity measures on the process. Quantum finite state machines are a natural model class for discussing similar issues in regard to systems with an underlying quantum dynamic. It is desired to understand the relationship between these models and computational mechanics - both by applying the known methods to the quantum processes, and by searching for a quantum analogue to the causal state. Finally, we wish to characterize the 'quantum effects' through information theory measures and show some suggestive results.
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• 12 June 2008 16:00 (3.34 Physics Department)
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• Markus Müller
• Convex trace functions on quantum channels and the additivity conjecture


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Abstract:
In quantum information theory, one of the most interesting open problems is the additivity conjecture. It states that the minimum output entropy of a pair of quantum channels should be the sum of the individual minimum entropies. A related problem is to determine if the minimum output p-norm for a bipartite channel is multiplicative. These problems can be seen as instances of a more general task: Determine the set of convex trace functions that attain their output maximum on unentangled input states. I report on some results and open problems for this generalized additivity problem. For example, I show that every operator convex function is "additive" in this sense for the Werner-Holevo channel, and give some speculation why von Neumann entropy and the p-"norms" for 1/2 < p < 1 might have special properties.
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• 11 June 2008 16:00 (3.34 Physics Department)
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• Maarten van den Nest (Max Planck Institute for Quantum Optics, Garching)
• Quantum algorithms for spin models and simulable gate sets for quantum computation


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Abstract:
We present elementary mappings between classical lattice models and quantum circuits. These mappings provide a general framework to obtain efficiently simulable quantum gate sets from exactly solvable classical models. For example, we recover and generalize the simulability of Valiant's match-gates by invoking the solvability of the free-fermion eight-vertex model. Our mappings furthermore provide a systematic formalism to obtain simple quantum algorithms to approximate partition functions of lattice models in certain complex-parameter regimes. For example, we present an efficient quantum algorithm for the six-vertex model as well as a 2D Ising-type model. We finally show that simulating our quantum algorithms on a classical computer is as hard as simulating universal quantum computation (i.e. BQP-complete).
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• 04 June 2008 16:00 (3.34 Physics Department)
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• Carlos A. Perez-Delgado (Sheffield)
• Quantizing Models of Computation


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Abstract:
The purpose of a model of computation is to abstract away any unnecessary details, in order to allow the mathematician/scientist to deal with important subject matter, unburdened. One lesson that quantum computation has taught us is that a detail we had previously thought unimportant --the nature of the underlying physical processes doing the computations-- is not only significant, but actually crucial in determining answers to many fundamental computational questions. As such, there has been a push to go back to our traditional, classical, computation models --e.g. Turing machines, finite and cellular automata-- and 'quantize' them. In this talk I will argue that this process is not always done properly. The reason being that our traditional models of computation are not actual physical theories; and quantizing anything but a physical theory can lead to nonsensical results. In my talk I will use the cellular automata model as the prime example of computational model quantizations. I will use this example to illustrate a quantization approach and practical advantages of quantizing models of computation using this approach.
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• 28 May 2008 16:00 (3.34 Physics Department)
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• Shashank Virmani (University of Hertfordshire)
• Upper bounds to fault tolerance thresholds for the common quantum computation architectures


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Abstract:
An important open problem in quantum information is determining fault tolerance thresholds for various quantum computation architectures - i.e. the amount of noise physical components can tolerate before they become incapable of efficiently simulating quantum computation. Much work has been done on constructing clever encoding mechanisms that allow the designers to prove lower bounds on these thresholds. However, relatively little work has been done on constructing upper bounds. I present some elementary extensions of previous work on this topic, which allow the construction of tighter upper bounds for architectures based on clifford operations plus other single qubit/unitary resources, in particular for `state injection' schemes.
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• 21 May 2008 16:00 (3.34 Physics Department)
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• Tony Sudbery (York)
• Mutually Unbiased Bases


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Abstract:
A set of mutually unbiased bases of a quantum Hilbert space corresponds to a set of measurements, such that if two of them are performed, the result of the first provides minimal information about the possible results of the second. Such measurements are the most efficient way of determining the state if many copies are available. In a space of dimension d, at most d + 1 mutually unbiased bases can exist, but this number is known to exist only if d is a power of a prime. In this talk I will review the theory and survey the available evidence about the existence of mutually unbiased bases in non-prime-power dimensions.
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• 13 May 2008 16:00 (3.34 Physics Department)
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• Jon Yard (Los Alamos National Labs)
• Hyperactivity of the quantum capacity


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Abstract:
The quantum capacity of a quantum channel is the minimum number of qubits which can be transmitted per use of the channel, with arbitrarily small error, in the limit of many uses. An easily computable formula for the capacity is only known for special classes of channels, while only upper and lower bounds are known in general. In this talk, I will prove that the quantum capacity is not additive by showing the existence of two channels, each of which has zero capacity, while the capacity of their tensor product is strictly positive. This is joint work with Graeme Smith (IBM Watson Laboratory).
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• 16 April 2008 16:00 (SM3 Mathematics Department)
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• Dan Browne (UCL)
• Measurement-based classical computation: Classifying the computational power of entangled states


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Abstract:
Measurement-based quantum computation has shown us that entangled states can in some sense be considered to have "computational power". In classical computer science, the notion of computational power has been successfully formalised in the field of computational complexity theory. In this talk, I will describe an approach to the classification of the computational power of families of entangled states in measurement-based quantum computation. This will lead naturally to the notion of measurement based "classical computation", in our analysis of which, familiar states and constructions will arise. In our analysis, we will draw on concepts and techniques from computational complexity theory and quantum foundations (such as non-signalling non-locality) and illustrate some unexpected connections between them. Joint work with Janet Anders (UCL) The talk will be accessible to all with a general background in quantum information - computer science jargon will be explained! Reference: J. Anders and D.E. Browne - Measurement-based classical computation - on the arXiv next week or soon afterwards...
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• 26 March 2008 16:00 (SM3 Mathematics Department)
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• Rob Spekkens (Cambridge)
• Contextuality from an information-theoretic perspective


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Abstract:
The Bell-Kochen-Specker theorem demonstrates that the predictions of quantum theory are inconsistent with a noncontextual hidden variable model. Significantly, the notion of noncontextuality appearing in this theorem is only well-defined for models of quantum theory (and then only for projective measurements and deterministic models). By contrast, the notion of locality has not been so restricted in its scope -- Bell's definition of a local hidden variable model applies to any theory that can be described operationally. In this talk, I present a novel definition of noncontextuality that applies to any operational theory, and recovers (modulo some improvements) the traditional notion in the quantum context. With this definition, one can derive "contextuality inequalities" for experimental statistics, constraints that are implied by the existence of a noncontextual hidden variable model of those statistics (and violated by quantum theory). In this way, one can determine whether contextuality-inequality-violating theories provide information-theoretic advantages over their noncontextual counterparts. I demonstrate that this is indeed the case by showing that contextuality powers a certain kind of multiplexing scheme (joint work with Ben Toner). Time permitting, I will discuss an experiment by the group of Geoff Pryde that confirms the existence of this advantage.
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• 12 March 2008 16:30 (SM3 Mathematics Department)
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• Tobias Osborne (Royal Holloway)
• Information propagation through disordered systems


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Abstract:
In 1972 Lieb and Robinson proved a bound on the velocity of propagation of correlations through an interacting spin system. This bound, when phrased in slightly more modern terms, says that all information, both classical and quantum, is exponentially attenuated outside of a light cone with an effective speed of light determined by the lattice structure. In this talk I will discuss Lieb-Robinson type bounds for systems with static and dynamical disorder. It turns out that, when disorder is present, the Lieb-Robinson bound can be replaced by a stronger bound. The bounds that arise depend on the nature and strength of the disorder and at least three types of behaviour can be identified: information propagates either ballistically, diffusively, or is strongly localised. Implications for fault tolerance in quantum computers will be sketched.
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• 05 March 2008 16:00 (SM3 Mathematics Department)
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• Rochus Klesse
• Quantum Error-Correction in correlated quantum noise


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Abstract:
Quantum error-correction and fault-tolerant computation rely crucially on the statistical independence of the noise that affects qubits at different times and locations. Within an n-spin-boson model we have quantitatively investigated to which extend this assumption is fulfilled in typical physical settings. I will present some evidence that noise-correlations that are introduced by the exchange of bosons between spins can drastically reduce the performance of QEC.
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• 22 February 2008 13:00 (1.06 Merchant Venturers Building)
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• Jeff Lundeen (Oxford)
• The search for the perfect photon
• Joint with photonics group


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Abstract:
Indistinguishable single photons in pure quantum states are a cornerstone of theoretical quantum optics. However, experimentally, it is not trivial to prepare them. For example, spontaneous parametric downconversion (SPDC) is commonly used to create heralded single photons but generally emits photon pairs into multiple correlated spatio-temporal modes, which introduces frequency and time jitter. I will explain how by careful design of the space–time modes available to the photons generated by SPDC in a nonlinear crystal, we have prepared nearly indistinguishable heralded photons with a purity of at least 95%, the highest yet reported. Since the technique also raises the photon generation rate and heralding efficiency, and since purity is the current limiting factor for logic gate fidelity, this source should be very useful for photonic quantum information processing. Moreover, the design principle behind it can be widely applied. I will report on our theoretical and experimental progress in extending it to photon generation via four-wave mixing in photonic crystal fibre. For reasons I will explain, these sources should also prove uniquely useful for generating squeezed light. As well as generating quantum light, we are also active in developing new ways to detect it. To accurately characterize or reconstruct a quantum state one must first have a detector that is also well characterized. I will describe the first experimental detector tomography and present the measured POVMs of a standard photon detector and our home built photon-number detector. The latter is revealed to be a better single-photon detector than the standard photon detector in some respects.
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• 19 February 2008 16:00 (1.06 Merchant Venturers Building)
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• Fernando Brandao (Imperial College)
• A reversible theory of entanglement and its relation to the second law


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Abstract:
Entanglement is central both to the foundations of quantum theory and, as a novel resource, to quantum information science. The theory of entanglement establishes basic laws, such as the non-increase of entanglement under local operations, that govern its manipulation and aims to draw from them formal analogies to thermodynamics and the second law. However, while in the second law the entropy uniquely determines whether a state is adiabatically accessible from another, the manipulation of entanglement under local operations exhibits a fundamental irreversibility which prevents the existence of such an order. In this talk it will be shown that a reversible theory of entanglement and a rigorous relationship with thermodynamics may be established when one considers all non-entangling transformations. The role of entropy and the second law is taken by the asymptotic relative entropy of entanglement and the basic law of entanglement. The usefulness of this new approach to general resource theories and to quantum information theory will be discussed. The talk is based on a joint work with Martin Plenio.
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• 13 February 2008 16:00 (SM3 Mathematics Department)
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• David W. Lyons (Lebanon Valley College)
• Maximum Stabilizer Dimension for Multiparty States


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Abstract:
This talk will present recent results in the classification of entanglement types for pure n-qubit states. We illustrate a method of analysis that uses the Lie algebra of the group of local unitary transformations. Stabilizer subalgebra dimension and isomorphism class are invariants that separate entanglement types. High stabilizer dimension is associated with states with interesting entanglement properties.
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• 06 February 2008 16:00 (SM3 Mathematics Department)
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• Aram Harrow (Bristol)
• Quantum expanders from any classical Cayley graph expander


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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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• 30 January 2008 14:00 (SM2 Mathematics Department)
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• Toby Cubitt (Bristol)
• The War on Additivity


(view abstract)
Abstract:
As you've no doubt already heard, a small band of brothers headed up by Lieutenant Winter scored dramatic victories in the war on additivity at the end of last year. But so far, you've only heard incomplete, censored information disseminated by the press and propaganda units. For the first time in Bristol, I will be giving a detailed, in depth report straight from the battle-front. Two of the big additivity questions of quantum information - additivity of the classical channel capacity and additivity of the entanglement of formation - are equivalent to additivity of the minimum output von Neumann entropy. The most popular way to attack (or defend) additivity is to consider the minimum output entropy problem for the more general p-Renyi entropies, where we recover the key von Neumann case at p=1. I will explain how Lieutenant Winter mounted a daring campaign against p>2, which Captain Hayden quickly followed up with his own attack, turning this initial breach into an astounding victory over all p>1. Additivity managed to regroup and retrench around p=1, and threw off the final assault, and a stand-off ensued. Mustering fresh forces, including NCOs Leung, Harrow and Montanaro and Corporal Cubitt, Lieutenant Winter organized a fresh attack on additivity from a new direction, which met with initial success at p=0, though for now seems to have become bogged down in difficult terrain.
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• 25 January 2008 13:00 (1.06 Merchant Venturers Building)
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• Chris Fuchs (Perimeter Institute)
• Charting the Shape of Hilbert Space: A Bit of Quantum Foundations at PI


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Abstract:
As physicists, we have become accustomed to the idea that a theory's content is always most transparent when written in coordinate-free language. But sometimes the choice of a good coordinate system is very useful for settling deep conceptual issues. Think of how Eddington-Finkelstein coordinates settled the longstanding question of whether the event horizon of a Schwarzschild black hole corresponds to a real spacetime singularity or not. Similarly we believe for an information-oriented or Bayesian approach to quantum foundations: That one good coordinate system may (eventually!) be worth more than a hundred blue-in-the-face arguments. This talk will motivate and chronicle the search for one such class of coordinate systems---the so-called Symmetric Informationally Complete Measurements---which has caught the attention of a handful of us at PI, a handful of our visitors, and a handful of other colleagues around the world.
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• 23 January 2008 16:00 (3.34 H.H. Wills Physics Laboratory)
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• Tim Spiller (HP Labs Bristol)
• The quantum-classical crossover of a field mode
• Joint with Theoretical Physics Group Seminar


• 16 January 2008 16:00 (SM3 Mathematics Department)
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• Jonathan Walgate (Perimeter Institute)
• Completely Entangled Random Subspaces


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Abstract:
We show how a simple fact of algebraic geometry translates into quantum information theory, determining when subspaces of multipartite Hilbert space are generically completely entangled. This in turn implies other generic results concerning the local distinguishability of random quantum states, the separability of random mixed states, the indistinguishability of random subspaces, and a mathematically interesting generalisation of Schmidt rank. Joint work with Andrew Scott.
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• 12 December 2007 16:00 (SM3 Mathematics Department)
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• Anthony Laing
• Experimental Demonstration of Measurement ID with Photonic Qubits


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Abstract:
The fact that non-orthogonal states cannot be distinguished with certainty is well known in quantum information. It may then be surprising to learn that non-orthogonal measurement bases can be distinguished with certainty if entanglement is used. In this talk I will give details of the first experiment to realise this use of entanglement - an experiment that uses a pair of photons generated from a Spontaneous Parametric Down Conversion Source.
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• 06 December 2007 16:00 (1.8 Queens Building)
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• Alberto Politi
• Experimental demonstration of an Integrated Optical Quantum Gate


• 05 December 2007 16:00 (SM3 Mathematics Department)
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• Renato Renner (ETH Zurich)
• On the difficulty of extracting randomness from partially untrusted quantum devices


(view abstract)
Abstract:
Randomness is a crucial resource in cryptography. It has been suggested that "good" randomness could be generated in an "unconditionally secure" way using quantum devices, e.g., a beam splitter followed by two photodetectors (and commercial devices have actually been built). In this talk, however, I show that in a model where all quantum devices are (only slightly) untrusted, it is generally impossible to generate randomness that is useful for cryptographic applications.
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• 28 November 2007 16:00 (SM3 Mathematics Department)
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• David Gross (Imperial)
• Quantum Margulis expanders


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Abstract:
Expander graphs are combinatorial objects which play an important role in classical computer science. A graph is an expander if it is simultaneously sparse and highly connected. Expanders seem to capture essential properties of random graphs: they often come into play when one is concerned with a property which "typically" holds, but defies systematic understanding. Very recently there has been some interest in constructing quantum analogues of expander graphs. Using phase space methods, we present the perhaps simplest way to quantize the perhaps simplest classical expander: the one due to Margulis. The talk will be largely conceptual - focusing on the basic ideas behind expanders and quantum phase spaces.
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• 27 November 2007 14:00 (MVB 1.06)
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• Jürg Wullschleger
• Composable Security in the Bounded-Quantum-Storage Model
• "Enigma Variations" information security seminar


(view abstract)
Abstract:
As shown by Mayers, and Lo and Chau, even in the quantum setting it is impossible to achieve bit commitment and oblivious transfer in an unconditionally secure way. Damgaard et al. (FOCS '05, CRYPTO '07) showed how to securely implement bit commitment and oblivious transfer in a weaker setting called 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. We first give a simple attack that shows that these protocols (without any refinements to the model) are not composable. Then, we present a new, simulation-based definition for security in the bounded-quantum-storage model, and show that this definition allows for sequential composition of protocols. Finally, we prove the security of the 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. This is joint work with Stephanie Wehner.
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• 19 November 2007 16:00 (3.27 Physics Department)
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• Janet Anders
• Macroscopic entanglement, critical temperature and phase transitions


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Abstract:
I will talk about the entanglement properties of thermal equilibrium states of many-body systems, with focus on harmonic lattices and quantum fields. I show that entanglement exists on the macroscopic scale and how it can be detected using thermodynamic quantities such as energy and temperature. I establish the parameter regimes where entanglement is present and discuss their dependency on the system parameters. In the case of the Bosonic gas, I link the entanglement to the occurrence of Bose-Einstein condensation in three dimensions. Moreover, I discuss the similarities between previous order concepts and entanglement as a new order concept. Finally, I suggest an analogy between measurement-based quantum computing and the process of phase transition in the two-dimensional Ising model and speculate about the implications of the presented ideas.
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• 14 November 2007 16:00 (SM3 Mathematics Department)
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• Stephen Bartlett (Sydney)
• Quantum-computational universality and quantum phase transitions in the ground states of spin lattices


(view abstract)
Abstract:
The state of a quantum many-body system can serve as a universal resource for measurement-based quantum computing (MBQC). Such states may be created dynamically using entangling gates, but an alternate and possibly less demanding approach might be to cool a suitably gapped spin lattice with appropriate interactions close to its ground state. However, it would be unfortunate if the universality of such a ground state for the purposes of MBQC was critically dependent on the exact details of the Hamiltonian. A stronger result would be if its universality was robust over a finite range of parameters describing the Hamiltonian, such as the presence of local field terms, and for a finite temperature range. Could it be possible that, beyond simply the existence of a universal resource state for MBQC, there was an entire _phase_ in which any ground or low-temperature thermal state was universal for a fixed MBQC scheme? Using the cluster model (with Hamiltonian given by the sum of the stabilizers) and a local field term on one- and two-dimensional lattices, we provide evidence to support this idea. Specifically, we show the existence of phase transitions in the ground state, with long-ranged correlation functions for performing quantum gates serving as the order parameters.
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• 14 November 2007 13:00 (SM2 Mathematics Department)
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• Terry Rudolph (Imperial College)
• Do quantum computers exist in nature?


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Abstract:
The leading proposals for quantum computation, either in the circuit or measurement-based models, all require the "hard work" of generating entanglement to be done dynamically. I will discuss the possibility that many-body systems may have a ground state which has suitable entanglement for quantum computation already - so that preparing the system amounts to simply cooling it.
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• 13 November 2007 16:00 (3.27 Physics Department)
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• Geoff Pryde (Griffith)
• Entanglement-free, Heisenberg-limited phase estimation


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Abstract:
Measurement underpins all quantitative science, and advances in precision measurement often lead to new physical understanding. When technical noise is stripped away, the ultimate limit to measurement precision is set by the Heisenberg uncertainty principle. We demonstrate the first experimental Heisenberg-limited scaling of the phase variance in optical phase measurements. Our algorithm replaces complicated entangled states - widely thought to be essential for Heisenberg scaling - with single photons, multiple passes through a phase shift, and adaptive measurement. We show that although a single-photon version of Kitaev's phase estimation algorithm has standard-quantum-limited variance, our generalized technique is within a small constant factor of the ultimate quantum limit.
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• 07 November 2007 16:00 (SM3 Mathematics Department)
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• Marie Ericsson (Cambridge)
• Nodal free geometric phases


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Abstract:
I will talk about the geometric phase in quantum mechanics and some recent extensions. For a non cyclic evolution the geometric phase is undefined if the initial and final states are orthogonal. Similarly, the off-diagonal geometric phase is undefined for cyclic evolutions. We define a new kind of geometric phase which is always well defined and can be measured interferometrically. These nodal free geometric phases can also be used to construct various types of quantum phase gates.
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• 01 November 2007 16:00 (1.15 Queens Building)
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• Marco Barbieri (University of Queensland)
• Experimental test of Bell's original inequalilty
• Photonics Research Group Seminar Programme


(view abstract)
Abstract:
Bell's original inequality was based on the assumption of perfect anti-correlations. Because these are unobtainable in practice, it is often thought that the inequality is inadequate for testing local hidden variables in actual experiments. We show that this is not the case. We describe how to use both Bell's original inequality and a similar inequality for experimental tests of local hidden variables, despite both being in principle valid only for perfect anti-correlations. The second inequality can be used to demonstrate violation of local realism in conditions where commonly used inequalities fail. We implement both schemes experimentally, finding significant violations of both inequalities using polarisation-entangled photons.
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• 24 October 2007 16:00 (SM3 Mathematics Department)
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• Nadav Yoran
• Classical simulability and the components of Shor's algorithm


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Abstract:
I will give a short introduction to classical simulation of quantum computation. I will then explain the tensor contraction approach for simulating quantum circuits. I will show how the approximate quantum Fourier transform circuit can be efficiently simulated using tensor contraction. Finally I will discuss the possibility of simulating the circuit for modular exponentiation (the other main component in Shor's algorithm) and show that an efficient tensor contraction scheme simulating this circuit can also efficiently simulate Shor's algorithm.
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• 17 October 2007 16:00 (SM3 Mathematics Department)
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• Simon Benjamin (Oxford)
• Distributed entanglement generation: a promising route toward a practical quantum computer?


(view abstract)
Abstract:
Last month the Monroe group published a report in Nature describing a successful entanglement generation experiment. The novelty of experiment was that the physical qubits, two trapped ions, were located over a metre apart -- the entanglement was accomplished purely through making measurements on emitted photons. Using measurements as the basic mechanism for entanglement generation is an attractive idea, especially in terms of scalability. However this first experimental demonstration also highlighted some of the difficulties, not least the inherently probabilistic nature of the approach (in fact that only four in a billion attempts at entanglement succeeded). I will try to make that case that the QIP paradigm of optical measurements on remote matter qubits, whether they are atoms, NV centres or quantum dots, is nevertheless an exciting one -- perhaps even the most promising route to a real device.
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• 16 October 2007 16:00 (3.34 Physics Department)
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• Libby Heaney (Leeds)
• Entanglement in the Bose-Einstein condensate phase transition


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Abstract:
I will speak about spatial entanglement in the BEC phase transition. I will quantify the amount of entanglement between two spatial modes around the transition temperature for condensation and prove that long-range order is necessary for the existence of two-mode entanglement. I will compare the results for entanglement and long-range order to experimental measurements of spatial coherence and close agreement is found. Multi-mode entanglement also can be considered and I will illustrate the suitable conditions for three-mode entanglement.
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• 10 October 2007 16:00 (SM3 Mathematics Department)
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• Karoline Wiesner
• Intrinsic quantum computation


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Abstract:
How are computing, information creation, and dynamics related? A contemporary view of these three historical threads is that they are not so disparate. I will show that a synthesis leads to methods to analyze how quantum processes store and manipulate information---what we refer to as "intrinsic quantum computation". I will introduce ways to measure information storage in quantum dynamics, using a recently introduced computation-theoretic model that accounts for measurement effects. The first, the entropy rate, quantifies the amount of inherent randomness in a process. The second, the excess entropy, quantifies the shared information between a quantum process's past and its future. A few examples of physical systems show how information processing is embedded in even simple quantum systems' behavior.
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• 03 October 2007 16:00 (SM3 Mathematics Department)
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• Sougato Bose (UCL)
• Quantum Communication and Entanglement Distribution Through Spin Chains and Allied Systems


(view abstract)
Abstract:
I will start with a brief introduction to the use of spin chains for quantum communications. Following this, I will describe some methods to perfect these communications using certain reasonbable strategies when one uses chains in their ferromagnetic ground state. I will then briefly point out the use of antiferromagnetic spin chains for the same purpose. Next, I will discuss the use of bosons hopping freely in a one dimensional lattice to generate entanglement between its ends. I will also describe the generation of entanglement from the ground state of a chain of qudits coupled by purely exchange interactions.
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• 20 September 2007 16:30 (SM3 Mathematics Department)
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• Richard Low
• Random circuits as t-designs


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Abstract:
In this talk I will discuss some ongoing work on the convergence properties of random quantum circuits. A random circuit can be used to generate an approximate t-design for any t. In this work, we ask how many steps of the random circuit are required to do this. I will show that O(n^2) steps suffice for an approximate 2-design and will discuss approaches to proving similar results for higher t.
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• 20 September 2007 16:00 (SM3 Mathematics Department)
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• Jonathan Allcock
• Localization in Spin Chains and Quantum Error Correction


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Abstract:
An ideal spin chain with nearest-neighbour couplings can be used to communicate quantum information with high fidelity, and can therefore be used as a quantum wire. Unfortunately, any real spin chain will inevitably have an element of randomness inherent in the system. As a result, the chain will be affected by Anderson localization, a well studied phenomenon in solid state physics. Whereas a perfect lattice has energy eigenstates which extend throughout the system, randomness in the lattice Hamiltonian causes the energy eigenstates to become exponentially localized in space. This in turn inhibits state transfer and challenges the use of spin chains for quantum communication. In an attempt to mitigate the affects of localization in spin chains we shall consider sending the message states down multiple, parallel chains. By using quantum error correction at periodic intervals, we will find that it is possible to send a qubit over arbitrary distances, with arbitrarily good fidelity, using only a number of wires polylogarithmic in the distance propagated.
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• 13 September 2007 16:00 (Vice Chancellor's room, Queen's Building)
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• Dr. Neil Oxtoby (University of Hertfordshire)
• Realistic Quantum Trajectories and Probing Spin Baths


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Abstract:
I will summarise some recent work of mine on realistic quantum trajectories for a charge qubit. This realistic extension of quantum trajectory theory (continuous-in-time weak quantum measurement theory) accounts for experimental imperfections such as finite bandwidth and Johnson-Nyquist noise. It uses a more realistic signal to infer the qubit state as a function of time. If there is time, I will also present a very brief overview of my current research involving probing/characterising an environment of spins using single-qubit and double-qubit (entangled) probes.
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• 31 August 2007 16:00 (4.01 MVB)
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• Tim Ralph (University of Queensland)
• Single Photon Side-Bands


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Abstract:
Single photon states (and other non-Gaussian states) are typically studied in the time domain. In contrast, continuous variable Gaussian states such as squeezed states are typically studied at side-band frequencies. Much of modern optical communcation technology is also based on side-band techniques. In this talk I will discuss what it means to produce single photon states at side-band frequencies and propose several techniques for producing, analysing and using such states.
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• 31 August 2007 13:00 (4.01 MVB)
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• Andrew White (University of Queensland)
• Photonic quantum computing: Shor's algorithm and the road to fault-tolerance


• 15 August 2007 16:00 (SM4 Mathematics Department)
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• Tony Chefles (HP Labs)
• Capacities and distributed implementation of standard oracle operators


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Abstract:
The standard oracle operator corresponding to a function f is a unitary operator that computes this function coherently, i.e. it maintains superpositions. This operator acts on a bipartite system, where the subsystems are the input and output registers. In distributed quantum computation, these subsystems may be spatially separated, in which case we will be interested in its classical and entangling capacities. For an arbitrary function f, we show that the unidirectional classical and entangling capacities of this operator are log_2(n_f) bits/ebits, where n_f is the number of different values this function can take. An optimal procedure for bidirectional classical communication with a standard oracle operator corresponding to a permutation on Z_M is given. The bidirectional classical capacity of such an operator is found to be 2 log_2(M) bits. The proofs of these capacities are facilitated by an optimal distributed protocol for the implementation of an arbitrary standard oracle operator.
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• 07 August 2007 16:00 (4.01 Merchant Venturers Building)
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• André Stefanov (University of Vienna)
• Implementation of Simple Quantum Algorithms using Optical Cluster States


• 28 June 2007 11:00 (4.01 Merchant Venturers Building)
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• Andre Ahlbrecht (TU Braunschweig)
• Braid group representations and quantum computation


(view abstract)
Abstract:
The elements of a unitary braid group representation (uBR) can be considered as quantum operations. Some of these representations establish new methods for fault tolerant quantum computing and quantum algorithms. However, we focus on uBRs of Yang-Baxter type and consider associated quantum circuits. In that case one can translate braids into quantum circuits simply by replacing braid strands by quantum wires and braid crossings by a fixed quantum gate R acting on appropriate quantum systems. After exploring some representation classes, we show how to efficiently simulate quantum circuits arising from certain representations. For qubits we conclude that all uBRs of Yang-Baxter type can be simulated efficiently.
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• 27 June 2007 16:00 (SM3 Mathematics Department)
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• Nilhan Gurkan (Izmir)
• Two Qubit Entanglement in Heisenberg Magnetic Chains


(view abstract)
Abstract:
We study two qubit entanglement in the most general XYZ Heisenberg magnetic chain with (non)homogeneous magnetic fields and the DM anisotropic antisymmetric exchange interaction, arising from the spin-orbit coupling . The model includes all known results as particular cases, for both antiferromagnetic and ferromagnetic XX, XY, XXX, XXZ, XYZ chains. The concurrence of two qubit thermal entanglement and its dependence on anisotropic parameters, external magnetic field and temperature are studied in details. We found that in all cases, inclusion of the DM interaction, which is responsible for weak ferromagnetism in mainly antiferromagnetic crystals and spin arrangement in low symmetry magnets, creates (when it does not exist) or strengthens (when it exists) entanglement in XYZ spin chain. This implies existence of a relation between arrangement of spins and entanglement, in which the DM coupling plays an essential role. It suggests also that anisotropic antisymmetric exchange interaction could be an efficient control parameter of entanglement in the general XYZ case.
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• 20 June 2007 16:00 (SM4)
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• William Matthews (Bristol)
• Chernoff bound for the asymptotic LOCC discrimination of data hiding states


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Abstract:
In the recent seminar by Arleta Szkola, a Chernoff bound for symmetric quantum hypothesis testing was presented for the single party case. If the states to be discriminated are bipartite and the parties limited to LOCC operations things get significantly more complicated. For a particular family of data hiding states, we give the Chernoff bound by showing the optimal discimination strategy for any number of copies.
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• 06 June 2007 16:00 (SM4 Mathematics Department)
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• Arleta Szkola (Leipzig)
• Quantum Chernoff Distance


• 30 May 2007 16:00 (SM3 Mathematics Department)
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• Matthias Christandl (Cambridge)
• Comparing Non-Signalling Correlations


• 22 May 2007 16:00 (SM4 Mathematics Department)
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• Sandu Popescu (Bristol)
• Multiple-time states and measurements
• blackboard talk


• 10 May 2007 16:00 (TBA)
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• Vladimir Korepin (Stony Brook University)
• Quantum Algorithm for Partial Search.


(view abstract)
Abstract:
Searching and sorting used as a subroutine in many important algorithms. Quantum algorithm can find a target item in a database faster than any classical algorithm. One can trade accuracy for speed and find a part of the database (a block) containing the target item even faster, this is partial search. An example is the following: exact address of the target item is given by a sequence of many bits, but we need to know only some of them. More generally partial search considers the following problem: a database is separated into several blocks. We want to find a block with the target item, not the target item itself. I will explain a quantum partial search algorithm. It can be considered as a rotation in three dimensional real space. It is based on quant-ph/0504157, quant-ph/0503238, quant-ph/0503238, quant-ph/0608106, quant-ph/0609205
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• 09 May 2007 16:00 (SM4 Mathematics Department)
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• Jens Eisert (Imperial)
• Tensor networks and novel schemes for measurement-based quantum computing


• 25 April 2007 16:00 (SM3 Mathematics department)
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• Viv Kendon (Leeds)
• Optimal computation with noisy quantum walks


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Abstract:
Quantum versions of random walks on the line and cycle show a quadratic improvement in their spreading rate and mixing times respectively. The addition of decoherence to the quantum walk produces a more uniform distribution on the line, and even faster mixing on the cycle by removing the need for time-averaging to obtain a uniform distribution. By calculating the entanglement between the coin and the position of the quantum walker, the optimal decoherence rates are found to be such that all the entanglement is just removed by the time the final measurement is made. This requires only O(log T) random bits for a quantum walk of T steps.
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• 29 March 2007 16:00 (SM4 Mathematics Department)
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• Peter Turner (Calgary)
• Hidden degrees of freedom and distinguishability


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Abstract:
We present a method for describing and characterizing the state of N experimentally indistinguishable particles, that is to say particles that cannot be individually addressed due to experimental limitations. The technique relies upon a correct treatment of the exchange symmetry of the state among experimentally accessible and experimentally inaccessible degrees of freedom. Our technique is of direct relevance to current experiments in quantum optics, and my aim is to make the mathematics involved less mysterious so that the physics might be better appreciated.
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• 21 March 2007 16:00 (SM4 Mathematics department)
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• Tom Toffoli (Boston University)
• 'Life' without batteries is possible, can we afford it?


• 13 March 2007 13:00 (MVB 1.06)
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• Andrew Childs (Caltech)
• Quantum algorithms for hidden nonlinear structures


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Abstract:
One of the major challenges in the theory of quantum computation is to develop new quantum algorithms. Much of the work on this question has focused on the nonabelian hidden subgroup problem (HSP), attempting to extend Shor's solution of the abelian HSP, the backbone of his efficient quantum algorithm for factoring integers. Unfortunately, these efforts have met with only limited success. In this talk, I will describe an alternative way of generalizing the success of Shor's algorithm. Shor's algorithm can be used to find hidden linear structures, so a natural generalization is to find hidden structures of higher degree. I will describe several problems involving hidden nonlinear structures. Typically, such problems have exponential classical query complexity, but polynomial quantum query complexity. Furthermore, some such problems can be solved in polynomial time by a quantum computer. This is joint work with Leonard Schulman and Umesh Vazirani.
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• 28 February 2007 16:00 (SM3 Mathematics Department)
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• Ashley Montanaro
• Quantum search of partially ordered sets


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Abstract:
In this talk, I will discuss the generalisation of quantum search of unstructured and totally ordered sets to search of partially ordered sets (posets). I'll show that quantum algorithms for this task can achieve at most a quadratic improvement in query complexity over classical algorithms, up to logarithmic factors; I'll also give quantum algorithms that almost achieve this optimal reduction in complexity. I'll explicitly describe a quantum algorithm for searching forest-like posets and an optimal O(m^(1/2))-query quantum algorithm for searching posets derived from m*m arrays sorted by rows and columns. This leads to an optimal O(n^(1/2))-query quantum algorithm for finding the intersection of two sorted lists of n integers.
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