University of Bristol Quantum Computation & Information Group

Prof Jeremy O'Brien
Department of Physics
University of Bristol
H. H. Wills Physics Laboratory
Royal Fort, Tyndall Avenue
Bristol BS8 1TL, U.K.
Department of Electrical and Electronic Engineering
University of Bristol
Queens Building
University Walk
Bristol BS8 1UB, U.K.
Phone: +44(0)117 331-5153
Fax: +44(0)117 954-5206
Email: jeremy.obrien (at) bristol.ac.uk
Web page
Recent publications:

  • A quantum delayed choice experiment
    Alberto Peruzzo, Peter J. Shadbolt, Nicolas Brunner, Sandu Popescu, Jeremy L. O'Brien
    22 May 2012

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

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

    Abstract:
    Integrated quantum photonics is an appealing platform for quantum information processing, quantum communication and quantum metrology. In all these applications it is necessary not only to be able to create and detect Fock states of light but also to program the photonic circuits that implements some desired logical operation. Here we demonstrate a reconfigurable controlled two-qubit operation on a chip using a multiwaveguide interferometer with a tunable phase shifter. We find excellent agreement between theory and experiment, with a 0.98 \pm 0.02 average similarity between measured and ideal operations.
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  • Photon Pair Generation in Silicon Micro-Ring Resonator and Enhancement via Reverse Bias
    Erman Engin, Damien Bonneau, Chandra M. Natarajan, Alex Clark, M. G. Tanner, R. H. Hadfield, Sanders N. Dorenbos, Val Zwiller, Kazuya Ohira, Nobuo Suzuki, Haruhiko Yoshida, Norio Iizuka, Mizunori Ezaki, Jeremy L. O'Brien, Mark G. Thompson
    22 April 2012

    Abstract:
    We experimentally demonstrate photon pair generation through spontaneous four-wave mixing in a silicon micro-ring resonator. We report a coincidence-to-accidental (CAR) ratio of 456\pm18 using the full width half maximum of the coincidence histogram as the integration window. In order to overcome the free-carrier related performance degradation we have investigated reverse biasing the ring. We show that this method improves the pair generation rate by a factor of up to 2.1.
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  • GaN directional couplers for integrated quantum photonics
    Yanfeng Zhang, Loyd McKnight, Erman Engin, Ian M. Watson, Martin J. Cryan, Erdan Gu, Mark G. Thompson, Stephane Calvez, Jeremy L. O'Brien, Martin D. Dawson
    20 February 2012

    Abstract:
    Large cross-section GaN waveguides are proposed as a suitable architecture to achieve integrated quantum photonic circuits. Directional couplers with this geometry have been designed with aid of the beam propagation method and fabricated using inductively coupled plasma etching. Scanning electron microscopy inspection shows high quality facets for end coupling and a well defined gap between rib pairs in the coupling region. Optical characterization at 800 nm shows single-mode operation and coupling-length-dependent splitting ratios. Two photon interference of degenerate photon pairs has been observed in the directional coupler by measurement of the Hong-Ou-Mandel dip with 96% visibility.
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  • Quantum interference and manipulation of entanglement in silicon wire waveguide quantum circuits
    Damien Bonneau, Erman Engin, Kazuya Ohira, Nob Suzuki, Haruhiko Yoshida, Norio Iizuka, Mizunori Ezaki, Chandra M. Natarajan, Michael G. Tanner, Robert H. Hadfield, Sanders N. Dorenbos, Val Zwiller, Jeremy L. O'Brien, Mark G. Thompson
    31 January 2012

    Abstract:
    Integrated quantum photonic waveguide circuits are a promising approach to realizing future photonic quantum technologies. Here, we present an integrated photonic quantum technology platform utilising the silicon-on-insulator material system, where quantum interference and the manipulation of quantum states of light are demonstrated in components orders of magnitude smaller than in previous implementations. Two-photon quantum interference is presented in a multi-mode interference coupler, and manipulation of entanglement is demonstrated in a Mach-Zehnder interferometer, opening the way to an all-silicon photonic quantum technology platform.
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  • Implementation of an iterative quantum order finding algorithm
    Enrique Martin Lopez, Anthony Laing, Thomas Lawson, Roberto Alvarez, Xiao-Qi Zhou, Jeremy L. O'Brien
    18 November 2011

    Abstract:
    Quantum algorithms are computational routines that exploit quantum mechanics to solve problems exponentially faster than the best classical algorithms. The quantum order finding algorithm is a key example---it is the subroutine that delivers the exponential speed-up in Shor's factoring algorithm. To date, the demand on quantum resources---qubits and logic gates---even for small instances, has meant that there have been only four experimental realisations. We demonstrate a scalable, iterative order finding algorithm that uses one third the number of qubits in a scalable way. Encoding in higher-dimensional states, we implement a two-photon compiled algorithm for which the algorithmic output exhibits structure that is sensitive to noise, in contrast to previous demonstrations. These results point to larger-scale implementations of Shor's algorithm by harnessing substantial but scalable resource reductions applicable to all physical architectures.
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  • Guaranteed violation of a Bell inequality without aligned reference frames or calibrated devices
    Peter Shadbolt, Tamas Vertesi, Yeong-Cherng Liang, Cyril Branciard, Nicolas Brunner, Jeremy L. O'Brien
    09 November 2011

    Abstract:
    Bell tests---the experimental demonstration of a Bell inequality violation---are central to understanding the foundations of quantum mechanics, underpin quantum technologies, and are a powerful diagnostic tool for technological developments in these areas. To date, Bell tests have relied on careful calibration of the measurement devices and alignment of a shared reference frame between the two parties---both technically demanding tasks in general. Surprisingly, we show that neither of these operations are necessary, violating Bell inequalities with near certainty with (i) unaligned, but calibrated, measurement devices, and (ii) uncalibrated and unaligned devices. We demonstrate generic quantum nonlocality with randomly chosen local measurements on a singlet state of two photons implemented with reconfigurable integrated optical waveguide circuits based on voltage-controlled phase shifters. The observed results demonstrate the robustness of our schemes to imperfections and statistical noise. This new approach is likely to have important applications in both fundamental science and in quantum technologies, including device independent quantum key distribution.
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  • Calculating Unknown Eigenvalues with a Quantum Algorithm
    Xiao-Qi Zhou, Pruet Kalasuwan, Timothy C. Ralph, Jeremy L. O'Brien
    20 October 2011

    Abstract:
    Quantum algorithms are able to solve particular problems exponentially faster than conventional algorithms, when implemented on a quantum computer. However, all demonstrations to date have required already knowing the answer to construct the algorithm. We have implemented the complete quantum phase estimation algorithm for a single qubit unitary in which the answer is calculated by the algorithm. We use a new approach to implementing the controlled-unitary operations that lie at the heart of the majority of quantum algorithms that is more efficient and does not require the eigenvalues of the unitary to be known. These results point the way to efficient quantum simulations and quantum metrology applications in the near term, and to factoring large numbers in the longer term. This approach is architecture independent and thus can be used in other physical implementations.
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  • 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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  • Fast path and polarisation manipulation of telecom wavelength single photons in lithium niobate waveguide devices
    Damien Bonneau, Mirko Lobino, Pisu Jiang, Chandra M. Natarajan, Michael G. Tanner, Robert H. Hadfield, Sanders N. Dorenbos, Val Zwiller, Mark G. Thompson, Jeremy L. O'Brien
    19 July 2011

    Abstract:
    We demonstrate fast polarisation and path control of photons at 1550 nm in lithium niobate waveguide devices using the electro-optic effect. We show heralded single photon state engineering, quantum interference, fast state preparation of two entangled photons and feedback control of quantum interference. These results point the way to a single platform that will enable the integration of nonlinear single photon sources and fast reconfigurable circuits for future photonic quantum information science and technology.
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  • 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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  • Correlated photon-pair generation in a periodically poled MgO doped stoichiometric lithium tantalate reverse proton exchanged waveguide
    M. Lobino, G. D. Marshall, C. Xiong, A. S. Clark, D. Bonneau, C. M. Natarajan, M. G. Tanner, R. H. Hadfield, S. N. Dorenbos, T. Zijlstra, V. Zwiller, M. Marangoni, R. Ramponi, M. G. Thompson, B. J. Eggleton, J. L. O'Brien
    22 March 2011

    Abstract:
    We demonstrate photon-pair generation in a reverse proton exchanged waveguide fabricated on a periodically poled magnesium doped stoichiometric lithium tantalate substrate. Detected pairs are generated via a cascaded second order nonlinear process where a pump laser at wavelength of 1.55 $\mu$m is first doubled in frequency by second harmonic generation and subsequently downconverted around the same spectral region. Pairs are detected at a rate of 42 per second with a coincidence to accidental ratio of 0.7. This cascaded pair generation process is similar to four-wave-mixing where two pump photons annihilate and create a correlated photon pair.
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  • Nano-fabricated solid immersion lenses registered to single emitters in diamond
    L. Marseglia, J. P. Hadden, A. C. Stanley-Clarke, J. P. Harrison, B. Patton, Y.-L. D. Ho, B. Naydenov, F. Jelezko, J. Meijer, P. Dolan, J. M. Smith, J. G. Rarity, J. L. O'Brien
    07 December 2010

    Abstract:
    We describe a technique for fabricating micro- and nano-structures incorporating fluorescent defects in diamond with a positional accuracy in the hundreds of nanometers. Using confocal fluorescence microscopy and focused ion beam (FIB) etching we first locate a suitable defect with respect to registration marks on the diamond surface and then etch a structure using these coordinates. We demonstrate the technique here by etching an 8 micron diameter hemisphere positioned such that a single negatively charged nitrogen-vacancy defect lies at its origin. This type of structure increases the photon collection efficiency by removing refraction and aberration losses at the diamond-air interface. We make a direct comparison of the fluorescence photon count rate before and after fabrication and observe an 8-fold increase due to the presence of the hemisphere.
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  • Generation of correlated photon pairs in a chalcogenide As2S3 waveguide
    C. Xiong, G. D. Marshall, A. Peruzzo, M. Lobino, A. S. Clark, D.-Y. Choi, S. J. Madden, C. M. Natarajan, M. G. Tanner, R. H. Hadfield, S. N. Dorenbos, T. Zijlstra, V. Zwiller, M. G. Thompson, J. G. Rarity, M. J. Steel, B. Luther-Davies, B. J. Eggleton, J. L. O'Brien
    09 November 2010

    Abstract:
    We demonstrate the first 1550 nm correlated photon-pair source in an integrated glass platform-a chalcogenide As2S3 waveguide. A measured pair coincidence rate of 80 per second was achieved using 57 mW of continuous-wave pump. The coincidence to accidental ratio was shown to be limited by spontaneous Raman scattering effects that are expected to be mitigated by using a pulsed pump source.
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  • Coherence properties of a single dipole emitter in diamond
    Graham D. Marshall, Torsten Gaebel, Jonathan C. F. Matthews, Jorg Enderlein, Jeremy L. O'Brien, James R. Rabeau
    14 September 2010

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

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

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

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

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

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

    Abstract:
    The efficiency of collecting photons from optically active defect centres in bulk diamond is greatly reduced by refraction and reflection at the diamond-air interface. We report on the fabrication and measurement of a geometrical solution to the problem; integrated solid immersion lenses (SILs) etched directly into the surface of diamond. An increase of a factor of 10 was observed in the saturated count-rate from a single negatively charged nitrogen-vacancy (NV-) within a 5um diameter SIL compared with NV-s under a planar surface in the same crystal. A factor of 3 reduction in background emission was also observed due to the reduced excitation volume with a SIL present. Such a system is potentially scalable and easily adaptable to other defect centres in bulk diamond.
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  • 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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  • Operating quantum waveguide circuits with superconducting single-photon detectors
    C. M. Natarajan, A. Peruzzo, S. Miki, M. Sasaki, Z. Wang, B. Baek, S. Nam, R. H. Hadfield, J. L. O'Brien
    25 March 2010
    Appl. Phys. Lett. 96, 211101 (2010)

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

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

    Abstract:
    The first quantum technology, which harnesses uniquely quantum mechanical effects for its core operation, has arrived in the form of commercially available quantum key distribution systems that achieve enhanced security by encoding information in photons such that information gained by an eavesdropper can be detected. Anticipated future quantum technologies include large-scale secure networks, enhanced measurement and lithography, and quantum information processors, promising exponentially greater computation power for particular tasks. Photonics is destined for a central role in such technologies owing to the need for high-speed transmission and the outstanding low-noise properties of photons. These technologies may use single photons or quantum states of bright laser beams, or both, and will undoubtably apply and drive state-of-the-art developments in photonics.
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  • Reference frame independent quantum key distribution
    Anthony Laing, Valerio Scarani, John G. Rarity, Jeremy L. O'Brien
    05 March 2010

    Abstract:
    We describe a quantum key distribution protocol based on pairs of entangled qubits that generates a secure key between two partners in an environment of unknown and slowly varying reference frame. A direction of particle delivery is required, but the phases between the computational basis states need not be known or fixed. The protocol can simplify the operation of existing setups and has immediate applications to emerging scenarios such as earth-to-satellite links and the use of integrated photonic waveguides. We compute the asymptotic secret key rate for a two-qubit source, which coincides with the rate of the six-state protocol for white noise. We give the generalization of the protocol to higher-dimensional systems and detail a scheme for physical implementation in the three dimensional qutrit case.
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  • Experimental feedback control of quantum systems using weak measurements
    G. G. Gillett, R. B. Dalton, B. P. Lanyon, M. P. Almeida, M. Barbieri, G. J. Pryde, J. L. O'Brien, K. J. Resch, S. D. Bartlett, A. G. White
    20 November 2009
    Phys. Rev. Lett. 104, 080503 (2010)

    Abstract:
    A goal of the emerging field of quantum control is to develop methods for quantum technologies to function robustly in the presence of noise. Central issues are the fundamental limitations on the available information about quantum systems and the disturbance they suffer in the process of measurement. In the context of a simple quantum control scenario--the stabilization of non-orthogonal states of a qubit against dephasing--we experimentally explore the use of weak measurements in feedback control. We find that, despite the intrinsic difficultly of implementing them, weak measurements allow us to control the qubit better in practice than is even theoretically possible without them. Our work shows that these more general quantum measurements can play an important role for feedback control of quantum systems.
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  • 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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  • The entanglement beam splitter: a quantum-dot spin in a double-sided optical microcavity
    C.Y. Hu, W.J. Munro, J.L.O'Brien, J.G. Rarity
    23 October 2009
    Phys. Rev. B 80, 205326 (2009)

    Abstract:
    We propose an entanglement beam splitter (EBS) using a quantum-dot spin in a double-sided optical microcavity. In contrast to the conventional optical beam splitter, the EBS can directly split a photon-spin product state into two constituent entangled states via transmission and reflection with high fidelity and high efficiency (up to 100 percent). This device is based on giant optical circular birefringence induced by a single spin as a result of cavity quantum electrodynamics and the spin selection rule of trion transition (Pauli blocking). The EBS is robust and it is immune to the fine structure splitting in a realistic quantum dot. This quantum device can be used for deterministically creating photon-spin, photon-photon and spin-spin entanglement as well as a single-shot quantum non-demolition measurement of a single spin. Therefore, the EBS can find wide applications in quantum information science and technology.
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  • Violation of the Leggett-Garg inequality with weak measurements of photons
    M. E. Goggin, M. P. Almeida, M. Barbieri, B. P. Lanyon, J. L. O'Brien, A. G. White, G. J. Pryde
    13 July 2009

    Abstract:
    By weakly measuring the polarization of a photon between two strong polarization measurements, we experimentally investigate the correlation between the appearance of anomalous values in quantum weak measurements, and the violation of realism and non-intrusiveness of measurements. A quantitative formulation of the latter concept is expressed in terms of a Leggett-Garg inequality for the outcomes of subsequent measurements of an individual quantum system. We experimentally violate the Leggett-Garg inequality for several measurement strengths. Furthermore, we experimentally demonstrate that there is a one-to-one correlation between achieving strange weak values and violating the Leggett-Garg inequality.
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  • An Entanglement Filter
    Ryo Okamoto, Jeremy L. O'Brien, Holger F. Hofmann, Tomohisa Nagata, Keiji Sasaki, Shigeki Takeuchi
    04 May 2009
    Science 323, 483 (2009)

    Abstract:
    The ability to filter quantum states is a key capability in quantum information science and technology, in which one-qubit filters, or polarizers, have found wide application. Filtering on the basis of entanglement requires extension to multi-qubit filters with qubit-qubit interactions. We demonstrated an optical entanglement filter that passes a pair of photons if they have the desired correlations of their polarization. Such devices have many important applications to quantum technologies.
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  • 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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  • Robust photon-spin entangling gate using a quantum-dot spin in a microcavity
    C.Y.Hu, W.J.Munro, J.L.O'Brien, J.G. Rarity
    27 January 2009

    Abstract:
    Semiconductor quantum dots (known as artificial atoms) hold great promise for solid-state quantum networks and quantum computers. To realize a quantum network, it is crucial to achieve light-matter entanglement and coherent quantum-state transfer between light and matter. Here we present a robust photon-spin entangling gate with high fidelity and high efficiency (up to 50 percent) using a charged quantum dot in a double-sided microcavity. This gate is based on giant circular birefringence induced by a single electron spin, and functions as an optical circular polariser which allows only one circularly-polarized component of light to be transmitted depending on the electron spin states. We show this gate can be used for single-shot quantum non-demolition measurement of a single electron spin, and can work as an entanglement filter to make a photon-spin entangler, spin entangler and photon entangler as well as a photon-spin quantum interface. This work allows us to make all building blocks for solid-state quantum networks with single photons and quantum-dot spins.
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  • Understanding photonic quantum-logic gates: The road to fault tolerance
    Till J. Weinhold, Alexei Gilchrist, Kevin J. Resch, Andrew C. Doherty, Jeremy L. O'Brien, Geoffrey J. Pryde, Andrew G. White
    07 August 2008

    Abstract:
    Fault-tolerant quantum computing requires gates which function correctly despite the presence of errors, and are scalable if the error probability-per-gate is below a threshold value. To date, no method has been described for calculating this probability from measurements on a gate. Here we introduce a technique enabling quantitative benchmarking of quantum-logic gates against fault-tolerance thresholds for any architecture. We demonstrate our technique experimentally using a photonic entangling-gate. The relationship between experimental errors and their quantum logic effect is non-trivial: revealing this relationship requires a comprehensive theoretical model of the quantum-logic gate. We show the first such model for any architecture, and find multi-photon emission--a small effect previously regarded as secondary to mode-mismatch--to be the dominant source of logic error. We show that reducing this will move photonic quantum computing to within striking distance of fault-tolerance.
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  • Cavity Enhanced Spin Measurement of an NV Centre in Diamond
    A. Young, C.Y. Hu, L. Marseglia, J.P. Harrison, J.L. O'Brien, J.G. Rarity
    02 July 2008

    Abstract:
    We propose a high efficiency high fidelity measurement of the ground state spin of a single NV center in diamond, using the effects of cavity quantum electrodynamics. The scheme we propose is based in the one dimensional atom or Purcell regime, removing the need for high Q cavities that are challenging to fabricate. The ground state of the NV center consists of three spin levels $^{3}A_{(m=0)}$ and $^{3}A_{(m=\pm1)}$ (the $\pm1$ states are near degenerate in zero field). These two states can undergo transitions to the excited ($^{3}E$) state, with an energy difference of $\approx7-10$ $\mu$eV between the two. By choosing the correct Q factor, this small detuning between the two transitions results in a dramatic change in the intensity of reflected light. We show the change in reflected intensity can allow us to read out the ground state spin using a low intensity laser with an error rate of $\approx5.5\times10^{-3}$, when realistic cavity and experimental parameters are considered. Since very low levels of light are used to probe the state of the spin we limit the number of florescence cycles, thereby limiting the non spin preserving transitions through the intermediate singlet state $^{1}A$.
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  • A deterministic optical quantum computer using photonic modules
    A. M. Stephens, Z. W. E. Evans, S. J. Devitt, A. D. Greentree, A. G. Fowler, W. J. Munro, J. L. O'Brien, Kae Nemoto, L. C. L. Hollenberg
    26 May 2008
    PRA 78, 032318 (2008)

    Abstract:
    The optical quantum computer is one of the few experimental systems to have demonstrated small scale quantum information processing. Making use of cavity quantum electrodynamics approaches to operator measurements, we detail an optical network for the deterministic preparation of arbitrarily large two-dimensional cluster states. We show that this network can form the basis of a large scale deterministic optical quantum computer that can be fabricated entirely on chip.
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  • Quantum computing using shortcuts through higher dimensions
    B. P. Lanyon, M. Barbieri, M. P. Almeida, T. Jennewein, T. C. Ralph, K. J. Resch, G. J. Pryde, J. L. O'Brien, A. Gilchrist, A. G. White
    03 April 2008

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

    Abstract:
    Quantum metrology promises greater sensitivity for optical phase measurements than could ever be achieved classically. Here we present a theory of the phase sensitivity for the general case where the detection probability is given by an $N$ photon interference fringe. We find that the phase sensitivity has a complex dependence on both the intrinsic efficiency of detection $\eta$ and the interference fringe visibility $V$. Most importantly, the phase that gives maximum phase sensitivity is in general not the same as the phase at which the slope of the interference fringe is a maximum, as has previously been assumed. We determine the parameter range where quantum enhanced sensitivity can be achieved. In order to illustrate these theoretical results, we perform a four photon experiment with $\eta=3/4$ and $V=82\pm6$% (an extension of our previous work [Science \textbf{316}, 726 (2007)]) and find a phase sensitivity 1.3 times greater than the standard quantum limit at a phase different to that which gives maximum slope of the interference fringe.
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  • Optical Quantum Computing
    Jeremy L. O'Brien
    12 March 2008
    Science, 318, 1467 (2007)

    Abstract:
    In 2001 all-optical quantum computing became feasible with the discovery that scalable quantum computing is possible using only single photon sources, linear optical elements, and single photon detectors. Although it was in principle scalable, the massive resource overhead made the scheme practically daunting. However, several simplifications were followed by proof-of-principle demonstrations, and recent approaches based on cluster states or error encoding have dramatically reduced this worrying resource overhead, making an all-optical architecture a serious contender for the ultimate goal of a large-scale quantum computer. Key challenges will be the realization of high-efficiency sources of indistinguishable single photons, low-loss, scalable optical circuits, high efficiency single photon detectors, and low-loss interfacing of these components.
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  • An All Optical Fibre Quantum Controlled-NOT Gate
    Alex S. Clark, Jeremie Fulconis, John G. Rarity, William J. Wadsworth, Jeremy L. O'Brien
    13 February 2008
    Phys. Rev. A 79, 030303(R) (2009)

    Abstract:
    We report the first experimental demonstration of an optical controlled-NOT gate constructed entirely in fibre. We operate the gate using two heralded optical fibre single photon sources and find an average logical fidelity of 90% and an average process fidelity of 0.83<F<0.91. On the basis of a simple model we are able to conclude that imperfections are primarily due to the photon sources, meaning that the gate itself works with very high fidelity.
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  • 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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  • Experimental Quantum Process Discrimination
    Anthony Laing, Terry Rudolph, Jeremy L. O'Brien
    25 January 2008
    Phys. Rev. Lett. 102, 160502 (2009)

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

    Abstract:
    We present and analyze a quantum key distribution protocol based on sending entangled multi-qubit states instead of single-qubit states as in the trail-blazing scheme by Bennett and Brassard (BB84). Since the qubits are sent individually, an eavesdropper is limited to access them one by one. In an intercept-resend attack on entangled two-qubit states, this fundamental restriction is shown to reduce the information of the eavesdropper to be less than one third compared with BB84. Strikingly, the information gain can be made to vanish if eavesdropping is applied to only one of the pairwise entangled qubits. For entangled states of many qubits, our protocol is expected to show even more pronounced, potentially exponential, advantages.
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  • Giant optical Faraday rotation induced by a single-electron spin in a quantum dot: Applications to entangling remote spins via a single photon
    C. Y. Hu, A. Young, J. L. O'Brien, W.J. Munro, J. G. Rarity
    16 August 2007
    Phys. Rev. B 78, 085307 (2008)

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

    Abstract:
    Precision measurements are important across all fields of science. In particular, optical phase measurements can be used to measure distance, position, displacement, acceleration and optical path length. Quantum entanglement enables higher precision than would otherwise be possible. We demonstrate an optical phase measurement with an entangled four photon interference visibility greater than the threshold to beat the standard quantum limit--the limit attainable without entanglement. These results open the way for new high-precision measurement applications.
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  • Optical Quadbit One-way Quantum Computation
    Jaewoo Joo, Peter L. Knight, Jeremy L. O'Brien, Terry Rudolph
    01 August 2007
    Phys. Rev. A 76, 052326 (2007)

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

    Abstract:
    The polarisation of a pair of photons in the same spatio-temporal mode represents a three-level quantum system, a qutrit. However, several of the key advantages of single photon polarisation qubits are lost when moving to this higher dimension. These advantages include easy arbitrary rotations, polarising elements and entangling operations. We present a technique based on measurement-induced nonlinearity that greatly extends the range of qutrit operations possible using linear optics. Furthermore, we use the nonlinearity to create the first qubit-qutrit entangled state which we fully characterise using quantum state tomography.
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  • The Photonic Module: an on-demand resource for photonic entanglement
    Simon J. Devitt, Andrew D. Greentree, Jeremy O'Brien, Lloyd C.L. Hollenberg
    18 June 2007
    Phys. Rev. A. 76, 052312 (2007)

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