University of Bristol Quantum Computation & Information Group

Dr Anthony Laing
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
Email: anthony.laing (at)
Web page
Recent publications:

  • Observation of quantum interference as a function of Berry's phase in a complex Hadamard optical network
    Anthony Laing, Thomas Lawson, Enrique Martín López, Jeremy L. O'Brien
    21 May 2012

    Emerging models of quantum computation driven by multi-photon quantum interference, while not universal, may offer an exponential advantage over classical computers for certain problems. Implementing these circuits via geometric phase gates could mitigate requirements for error correction to achieve fault tolerance while retaining their relative physical simplicity. We report an experiment in which a geometric phase is embedded in an optical network with no closed-loops, enabling quantum interference between two photons as a function of the phase.
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  • Implementation of an iterative quantum order finding algorithm
    Enrique Martin Lopez, Anthony Laing, Thomas Lawson, Roberto Alvarez, Xiao-Qi Zhou, Jeremy L. O'Brien
    18 November 2011

    Quantum algorithms are computational routines that exploit quantum mechanics to solve problems exponentially faster than the best classical algorithms. The quantum order finding algorithm is a key example---it is the subroutine that delivers the exponential speed-up in Shor's factoring algorithm. To date, the demand on quantum resources---qubits and logic gates---even for small instances, has meant that there have been only four experimental realisations. We demonstrate a scalable, iterative order finding algorithm that uses one third the number of qubits in a scalable way. Encoding in higher-dimensional states, we implement a two-photon compiled algorithm for which the algorithmic output exhibits structure that is sensitive to noise, in contrast to previous demonstrations. These results point to larger-scale implementations of Shor's algorithm by harnessing substantial but scalable resource reductions applicable to all physical architectures.
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  • Multimode quantum interference of photons in multiport integrated devices
    Alberto Peruzzo, Anthony Laing, Alberto Politi, Terry Rudolph, Jeremy L. O'Brien
    12 July 2010

    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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  • 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

    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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  • A simple scheme for expanding photonic cluster states for quantum information
    Pruet Kalasuwan, Gabriel Mendoza, Anthony Laing, Tomohisa Nagata, Jack Coggins, Mark Callaway, Shigeki Takeuchi, Andre Stefanov, Jeremy L. O'Brien
    24 March 2010
    JOSA B, Vol. 27, Issue 6, pp. A181-A184 (2010)

    We show how an entangled cluster state encoded in the polarization of single photons can be straightforwardly expanded by deterministically entangling additional qubits encoded in the path degree of freedom of the constituent photons. This can be achieved using a polarization--path controlled-phase gate. We experimentally demonstrate a practical and stable realization of this approach by using a Sagnac interferometer to entangle a path qubit and polarization qubit on a single photon. We demonstrate precise control over phase of the path qubit to change the measurement basis and experimentally demonstrate properties of measurement-based quantum computing using a 2 photon, 3 qubit cluster state.
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  • Reference frame independent quantum key distribution
    Anthony Laing, Valerio Scarani, John G. Rarity, Jeremy L. O'Brien
    05 March 2010

    We describe a quantum key distribution protocol based on pairs of entangled qubits that generates a secure key between two partners in an environment of unknown and slowly varying reference frame. A direction of particle delivery is required, but the phases between the computational basis states need not be known or fixed. The protocol can simplify the operation of existing setups and has immediate applications to emerging scenarios such as earth-to-satellite links and the use of integrated photonic waveguides. We compute the asymptotic secret key rate for a two-qubit source, which coincides with the rate of the six-state protocol for white noise. We give the generalization of the protocol to higher-dimensional systems and detail a scheme for physical implementation in the three dimensional qutrit case.
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  • Experimental Quantum Process Discrimination
    Anthony Laing, Terry Rudolph, Jeremy L. O'Brien
    25 January 2008
    Phys. Rev. Lett. 102, 160502 (2009)

    The problem of discriminating between unknown processes chosen from a finite set is experimentally shown to be possible even in the case of non-orthogonal processes. We demonstrate unambiguous deterministic quantum process discrimination (QPD) of non-orthogonal processes using properties of entanglement, additional known unitaries, or higher dimensional systems. Single qubit, qutrit and qudit ($d$=10) measurement and unitary processes acting on photons are discriminated with a confidence of $>97%$ in all cases.
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