University of Bristol Quantum Computation & Information Group

Dr Alberto Politi
Department of Physics
University of Bristol
H. H. Wills Physics Laboratory
Royal Fort, Tyndall Avenue
Bristol BS8 1TL, U.K.
Fax: +44(0)117 925-5624
Web page
Recent publications:

  • Reconfigurable controlled two-qubit operation on a quantum photonic chip
    H. W. Li, S. Przeslak, A. O. Niskanen, J. C. F. Matthews, A. Politi, P. Shadbolt, A. Laing, M. Lobino, M. G. Thompson, J. L. O'Brien
    19 May 2012

    Abstract:
    Integrated quantum photonics is an appealing platform for quantum information processing, quantum communication and quantum metrology. In all these applications it is necessary not only to be able to create and detect Fock states of light but also to program the photonic circuits that implements some desired logical operation. Here we demonstrate a reconfigurable controlled two-qubit operation on a chip using a multiwaveguide interferometer with a tunable phase shifter. We find excellent agreement between theory and experiment, with a 0.98 \pm 0.02 average similarity between measured and ideal operations.
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  • Measuring protein concentration with entangled photons
    Andrea Crespi, Mirko Lobino, Jonathan C. F. Matthews, Alberto Politi, Chris R. Neal, Roberta Ramponi, Roberto Osellame, Jeremy L. O'Brien
    15 September 2011

    Abstract:
    Optical interferometry is amongst the most sensitive techniques for precision measurement. By increasing the light intensity a more precise measurement can usually be made. However, in some applications the sample is light sensitive. By using entangled states of light the same precision can be achieved with less exposure of the sample. This concept has been demonstrated in measurements of fixed, known optical components. Here we use two-photon entangled states to measure the concentration of the blood protein bovine serum albumin (BSA) in an aqueous buffer solution. We use an opto-fluidic device that couples a waveguide interferometer with a microfluidic channel. These results point the way to practical applications of quantum metrology to light sensitive samples.
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  • Generating, manipulating and measuring entanglement and mixture with a reconfigurable photonic circuit
    P. J. Shadbolt, M. R. Verde, A. Peruzzo, A. Politi, A. Laing, M. Lobino, J. C. F. Matthews, J. L. O'Brien
    17 August 2011

    Abstract:
    Entanglement is the quintessential quantum mechanical phenomenon understood to lie at the heart of future quantum technologies and the subject of fundamental scientific investigations. Mixture, resulting from noise, is often an unwanted result of interaction with an environment, but is also of fundamental interest, and is proposed to play a role in some biological processes. Here we report an integrated waveguide device that can generate and completely characterize pure two-photon states with any amount of entanglement and arbitrary single-photon states with any amount of mixture. The device consists of a reconfigurable integrated quantum photonic circuit with eight voltage controlled phase shifters. We demonstrate that for thousands of randomly chosen configurations the device performs with high fidelity. We generate maximally and non-maximally entangled states, violate a Bell-type inequality with a continuum of partially entangled states, and demonstrate generation of arbitrary one-qubit mixed states.
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  • Simulating quantum statistics with entangled photons: a continuous transition from bosons to fermions
    Jonathan C. F. Matthews, Konstantinos Poulios, Jasmin D. A. Meinecke, Alberto Politi, Alberto Peruzzo, Nur Ismail, Kerstin Wörhoff, Mark G. Thompson, Jeremy L. O'Brien
    07 June 2011

    Abstract:
    In contrast to classical physics, quantum mechanics divides particles into two classes-bosons and fermions-whose exchange statistics dictate the dynamics of systems at a fundamental level. In two dimensions quasi-particles known as 'anyons' exhibit fractional exchange statistics intermediate between these two classes. The ability to simulate and observe behaviour associated to fundamentally different quantum particles is important for simulating complex quantum systems. Here we use the symmetry and quantum correlations of entangled photons subjected to multiple copies of a quantum process to directly simulate quantum interference of fermions, bosons and a continuum of fractional behaviour exhibited by anyons. We observe an average similarity of 93.6\pm0.2% between an ideal model and experimental observation. The approach generalises to an arbitrary number of particles and is independent of the statistics of the particles used, indicating application with other quantum systems and large scale application.
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  • Multimode quantum interference of photons in multiport integrated devices
    Alberto Peruzzo, Anthony Laing, Alberto Politi, Terry Rudolph, Jeremy L. O'Brien
    12 July 2010

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

    Abstract:
    Quantum walks of correlated particles offer the possibility to study large-scale quantum interference, simulate biological, chemical and physical systems, and a route to universal quantum computation. Here we demonstrate quantum walks of two identical photons in an array of 21 continuously evanescently-coupled waveguides in a SiOxNy chip. We observe quantum correlations, violating a classical limit by 76 standard deviations, and find that they depend critically on the input state of the quantum walk. These results open the way to a powerful approach to quantum walks using correlated particles to encode information in an exponentially larger state space.
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  • Heralded entanglement for quantum enhanced measurement with photons
    Jonathan C. F. Matthews, Alberto Politi, Damien Bonneau, Jeremy L. O'Brien
    28 May 2010

    Abstract:
    Generating quantum entanglement is not only an important scientific endeavour, but will be essential to realising the tremendous potential of quantum-enhanced technologies, in particular quantum-enhanced measurements with precision beyond classical limits. We report the heralded generation of multi-photon entanglement for quantum metrology using a reconfigurable integrated waveguide device in which projective measurement of auxiliary photons heralds the generation of path entangled states. From four and six photon inputs we heralded two- and four-photon"NOON" states-a superposition of N photons in two paths, which enable phase supersensitive measurements at the Heisenberg limit. Realistic devices will include imperfections and we demonstrate phase super- resolution with a state that is robust to photon loss. These results can be generalised to generate arbitrarily large entangled states of light for quantum metrology in an integrated optics architecture.
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  • High-fidelity operation of quantum photonic circuits
    Anthony Laing, Alberto Peruzzo, Alberto Politi, Maria Rodas Verde, Matthaeus Halder, Timothy C. Ralph, Mark G. Thompson, Jeremy L. O’Brien
    05 April 2010

    Abstract:
    We demonstrate photonic quantum circuits that operate at the stringent levels that will be required for future quantum information science and technology. These circuits are fabricated from silica-on-silicon waveguides forming directional couplers and interferometers. While our focus is on the operation of quantum circuits, to test this operation required construction of a spectrally tuned photon source to produce near-identical pairs of photons. We show non-classical interference with two photons and a two-photon entangling logic gate that operate with near-unit fidelity. These results are a significant step towards large-scale operation of photonic quantum circuits.
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  • Demonstration of unconditional one-way quantum computations for continuous variables
    Ryuji Ukai, Noriaki Iwata, Yuji Shimokawa, Seiji C. Armstrong, Alberto Politi, Jun-ichi Yoshikawa, Peter van Loock, Akira Furusawa
    28 January 2010

    Abstract:
    One-way quantum computation, realizable through measurements on a highly entangled state with no need for controlled unitary evolutions, is a very promising approach to fulfil the capabilities of quantum information processing. We demonstrate unconditional one-way quantum computation experiments using a linear cluster state of four entangled optical modes. A continuous quantum variable in phase space serves as the computational basis |x>. The key element of the continuous-variable scheme is that it does not rely on postselection or any probabilistic events; it works unconditionally, an advantage for scalability of quantum computation. We implement an important set of quantum operations in the optical phase space through one-way computation: Fourier rotations and squeezing. Though not sufficient, these linear transformations are necessary for universal quantum computation over continuous variables, and in our scheme, in principle, any such linear transformation can be unconditionally and deterministically applied to arbitrary single-mode quantum states. Hence our results provide a first demonstration of the fundamental components required for one-way quantum computation with continuous variables.
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  • Shor's quantum factoring algorithm on a photonic chip
    Alberto Politi, Jonathan C. F. Matthews, Jeremy L. O'Brien
    09 November 2009
    Science, 325, 1221 (2009)

    Abstract:
    Shor's quantum factoring algorithm finds the prime factors of a large number exponentially faster than any other known method a task that lies at the heart of modern information security, particularly on the internet. This algorithm requires a quantum computer a device which harnesses the `massive parallelism' afforded by quantum superposition and entanglement of quantum bits (or qubits). We report the demonstration of a compiled version of Shor's algorithm on an integrated waveguide silica-on-silicon chip that guides four single-photon qubits through the computation to factor 15.
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  • Manipulating multi-photon entanglement in waveguide quantum circuits
    Jonathan C. F. Matthews, Alberto Politi, Andre Stefanov, Jeremy L. O'Brien
    09 November 2009
    Nature Photonics, 3, 346-350 (2009)

    Abstract:
    On-chip integrated photonic circuits are crucial to further progress towards quantum technologies and in the science of quantum optics. Here we report precise control of single photon states and multi-photon entanglement directly on-chip. We manipulate the state of path-encoded qubits using integrated optical phase control based on resistive elements, observing an interference contrast of 98.2+/-0.3%. We demonstrate integrated quantum metrology by observing interference fringes with 2- and 4-photon entangled states generated in a waveguide circuit, with respective interference contrasts of 97.2+/-0.4% and 92+/-4%, sufficient to beat the standard quantum limit. Finally, we demonstrate a reconfigurable circuit that continuously and accurately tunes the degree of quantum interference, yielding a maximum visibility of 98.2+/- 0.9%. These results open up adaptive and fully reconfigurable photonic quantum circuits not just for single photons, but for all quantum states of light.
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  • Laser written waveguide photonic quantum circuits
    G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, J. L. O'Brien
    26 February 2009
    Optics Express, Vol. 17, No. 15, pp. 12546-12554, 20 July 2009.

    Abstract:
    We report photonic quantum circuits created using an ultrafast laser processing technique that is rapid, requires no lithographic mask and can be used to create three-dimensional networks of waveguide devices. We have characterized directional couplers--the key functional elements of photonic quantum circuits--and found that they outperform previous lithographically produced waveguide devices. We further demonstrate high-performance interferometers and an important multi-photon quantum interference phenomenon for the first time in integrated optics.This direct-write approach will enable the rapid development of sophisticated quantum optical circuits and their scaling into three-dimensions.
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  • Silica-on-Silicon Waveguide Quantum Circuits
    Alberto Politi, Martin J. Cryan, John G. Rarity, Siyuan Yu, Jeremy L. O'Brien
    04 February 2008
    Science 27 March (2008) (10.1126/science.1155441)

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