T. C. Ralph

University of Queensland , Brisbane, Queensland, Australia

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Publications (239)626 Total impact

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    ABSTRACT: We show that a set of optical memories can act as a reconfigurable linear optical network operating on frequency-multiplexed optical states. Our protocol is applicable to any quantum memories that employ off-resonant Raman transitions to store optical information in atomic spins. In addition to the configurability, the protocol also offers favourable scaling with an increasing number of modes where N memories can be configured to implement an arbitrary N-mode unitary operations during storage and readout. We demonstrate the versatility of this protocol by showing a example where cascaded memories are used to implement a conditional CZ gate.
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    A. P. Lund, T. C. Ralph, H. Jeong
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    ABSTRACT: Entangled coherent states are useful for various applications in quantum information processing but they are are sensitive to loss. We propose a scheme to generate distributed entangled coherent states over a lossy environment in such a way that the fidelity is independent of the losses at detectors heralding the generation of the entanglement. We compare our scheme with a previous one for the same purpose [Ourjoumtsev {\em et al.}, Nat. Phys. {\bf 5} 189 (2009)] and find parameters for which our new scheme results in superior performance.
    Physical Review A 10/2013; · 3.04 Impact Factor
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    ABSTRACT: We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and a certain class of non-Gaussian states. We show that any separation in the peaks of the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurements performed on the other subsystem indicates correlation between the corresponding quadratures, and hence nonzero discord. We also apply this method to non-Gaussian states that are prepared by overlapping a statistical mixture of coherent and vacuum states on a beam splitter. We experimentally demonstrate this technique by verifying nonzero quantum discord in a bipartite Gaussian non-Gaussian states.
    Journal of Physics B Atomic Molecular and Optical Physics 10/2013; 47(2). · 2.03 Impact Factor
  • N. A. McMahon, A. P. Lund, T. C. Ralph
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    ABSTRACT: Non-deterministic quantum noiseless linear amplifiers are a new technology with interest in both fundamental understanding and new applications. With a noiseless linear amplifier it is possible to perform tasks such as improving the performance of quantum key distribution and purifying lossy channels. Previous designs for noiseless linear amplifiers involving linear optics and photon counting are non-optimal because they have a probability of success lower than the theoretical bound given by the theory of generalised quantum measurement. This paper develops a theoretical model which reaches this limit. We calculate the fidelity and probability of success of this new model for coherent states and Einstein-Podolsky-Rosen (EPR) entangled states.
    07/2013; 89(2).
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    ABSTRACT: We pose a generalized Boson Sampling problem. Strong evidence exists that such a problem becomes intractable on a classical computer as a function of the number of Bosons. We describe a quantum optical processor that can solve this problem efficiently based on Gaussian input states, a linear optical network and non-adaptive photon counting measurements. All the elements required to build such a processor currently exist. The demonstration of such a device would provide the first empirical evidence that quantum computers can indeed outperform classical computers and could lead to applications.
  • International Quantum Electronics Conference; 05/2013
  • N. Walk, T. Symul, P.K. Lam, T.C. Ralph
    International Quantum Electronics Conference; 05/2013
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    ABSTRACT: We experimentally demonstrate the reconstruction of a photon number conditioned state without using a photon number discriminating detector. By using only phase randomized homodyne measurements, we reconstruct up to the three photon subtracted squeezed vacuum state. The reconstructed Wigner functions of these states show regions of pronounced negativity, signifying the non-classical nature of the reconstructed states. The techniques presented allow for complete characterization of the role of a conditional measurement on an ensemble of states, and might prove useful in systems where photon counting still proves technically challenging.
    Journal of Physics B Atomic Molecular and Optical Physics 05/2013; 46(10):104009. · 2.03 Impact Factor
  • P. K. Lam, T. C. Ralph
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    ABSTRACT: Using photons to disseminate encryption codes with complete security is one of the great successes of quantum information science. It has now been shown that long-distance cryptographic communication is just as effective when the scheme involves measuring the wave properties of light, rather than its particle properties.
    Nature Photonics 05/2013; 7(5):350-352. · 27.25 Impact Factor
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    T. G. Downes, T. C. Ralph, N. Walk
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    ABSTRACT: An unsolved problem in relativistic quantum information research is how to model efficient, directional quantum communication between localised parties in a fully quantum field theoretical framework. We propose a tractable approach to this problem based on solving the Heisenberg evolution of localized field observables. We illustrate our approach by analysing, and obtaining approximate analytical solutions to, the problem of communicating coherent states between an inertial sender, Alice and an accelerated receiver, Rob. We use these results to determine the efficiency with which continuous variable quantum key distribution could be carried out over such a communication channel.
    Physical Review A 03/2013; 87(1):012327. · 3.04 Impact Factor
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    J L Pienaar, T C Ralph, C R Myers
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    ABSTRACT: Toy models for quantum evolution in the presence of closed timelike curves have gained attention in the recent literature due to the strange effects they predict. The circuits that give rise to these effects appear quite abstract and contrived, as they require nontrivial interactions between the future and past that lead to infinitely recursive equations. We consider the special case in which there is no interaction inside the closed timelike curve, referred to as an open timelike curve (OTC), for which the only local effect is to increase the time elapsed by a clock carried by the system. Remarkably, circuits with access to OTCs are shown to violate Heisenberg's uncertainty principle, allowing perfect state discrimination and perfect cloning of coherent states. The model is extended to wave packets and smoothly recovers standard quantum mechanics in an appropriate physical limit. The analogy with general relativistic time dilation suggests that OTCs provide a novel alternative to existing proposals for the behavior of quantum systems under gravity.
    Physical Review Letters 02/2013; 110(6):060501. · 7.94 Impact Factor
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    ABSTRACT: The extended Church-Turing thesis posits that any computable function can be calculated efficiently by a probabilistic Turing machine. If this thesis held true, the global effort to build quantum computers might ultimately be unnecessary. The thesis would however be strongly contradicted by a physical device that efficiently performs a task believed to be intractable for classical computers. BosonSampling-the sampling from a distribution of n photons undergoing some linear-optical process-is a recently developed, and experimentally accessible example of such a task.
    Photonics Society Summer Topical Meeting Series, 2013 IEEE; 01/2013
  • Coherence and Quantum Optics(CQO-X); 01/2013
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    ABSTRACT: We present a quantum repeater scheme that uses gradient echo memory and probabilistic noiseless amplification. We show that a quantum memory can process quantum information and a noiseless amplifier can distill entanglement.
    Lasers and Electro-Optics Pacific Rim (CLEO-PR), 2013 Conference on; 01/2013
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    ABSTRACT: Quantum discord is conjectured to be a more general quantum resource than entanglement. We support this conjecture by showing, via experimental Gaussian optics, that quantum processors can harness discord to perform tasks classical counterparts cannot.
    Lasers and Electro-Optics Pacific Rim (CLEO-PR), 2013 Conference on; 01/2013
  • T.C. Ralph, N. Walk, T. Symul, P.K. Lam
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    ABSTRACT: Post-selection is a standard part of discrete variable QKD protocols, however attempts to prove security when post-selection is deployed in continuous variable QKD protocols have until now been limited. Here, by using a carefully tailored post-selection protocol, we prove unconditional security and show that significant performance improvements are achieved.
    Photonics Society Summer Topical Meeting Series, 2013 IEEE; 01/2013
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    R B Mann, T C Ralph
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    ABSTRACT: Over the past few years, a new field of high research intensity has emerged that blends together concepts from gravitational physics and quantum computing. Known as relativistic quantum information, or RQI, the field aims to understand the relationship between special and general relativity and quantum information.
    Classical and Quantum Gravity 11/2012; 29(22):220301. · 3.56 Impact Factor
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    ABSTRACT: Non-deterministic noiseless amplification of a single mode can circumvent the unique challenges to amplifying a quantum signal, such as the no-cloning theorem, and the minimum noise cost for deterministic quantum state amplification. However, existing devices are not suitable for amplifying the fundamental optical quantum information carrier, a qubit coherently encoded across two optical modes. Here, we construct a coherent two-mode amplifier, to demonstrate the first heralded noiseless linear amplification of a qubit encoded in the polarization state of a single photon. In doing so, we increase the transmission fidelity of a realistic qubit channel by up to a factor of five. Qubit amplifiers promise to extend the range of secure quantum communication and other quantum information science and technology protocols.
    Nature Physics 08/2012; 9(1). · 19.35 Impact Factor
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    D. Ahn, C. R. Myers, T. C. Ralph, R. B. Mann
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    ABSTRACT: The possible existence of closed timelike curves (CTCs) draws attention to fundamental questions about what is physically possible and what is not. An example is the "no cloning theorem" in quantum mechanics, which states that no physical means exists by which an unknown arbitrary quantum state can be reproduced or copied perfectly. Using the Deutsch approach, we have shown that the no-cloning theorem can be circumvented in the presence of closed timelike curves, allowing the perfect cloning of a quantum state chosen randomly from a finite alphabet of states. Further, we show that a universal cloner can be constructed that when acting on a completely arbitrary qubit state, exceeds the no-cloning bound on fidelity. Since the no cloning theorem has played a central role in the development of quantum information science, it is clear that the existence of closed timelike curves would radically change the rules for quantum information technology. Nevertheless, we show that this type of cloning does not violate no-signalling criteria.
    Physical Review A. 07/2012; 88(2).
  • T. C. Ralph
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    ABSTRACT: According to the Fiurás¿ek's optimal condition, where the average fidelity works as the figure of merit of quantum cloning machines (QCMs), the optimal cloning machine should maximize a convex mixture of the average fidelity. Based on this condition, ...
    Quantum Information Processing 01/2012; · 1.75 Impact Factor

Publication Stats

5k Citations
626.00 Total Impact Points


  • 2000–2013
    • University of Queensland 
      • • School of Mathematics and Physics
      • • Centre for Plant Science
      Brisbane, Queensland, Australia
  • 2011
    • Griffith University
      • Centre for Quantum Dynamics
      Southport, Queensland, Australia
  • 2010
    • Max-Planck-Institut für die Physik des Lichts
      Erlangen, Bavaria, Germany
  • 2004–2010
    • University of New South Wales
      • • Centre for Quantum Computer Technology (CQCT)
      • • School of Engineering and Information Technology
      Kensington, New South Wales, Australia
  • 1992–2007
    • Australian National University
      • Department of Quantum Science (DQS)
      Canberra, Australian Capital Territory, Australia
  • 2005
    • University of Canberra
      Canberra, Australian Capital Territory, Australia
  • 2002
    • Friedrich-Alexander Universität Erlangen-Nürnberg
      • Institute of Optics, Information and Photonics
      Erlangen, Bavaria, Germany
  • 1999
    • Shanxi University
      Yangkü, Shanxi Sheng, China
  • 1997
    • University of Auckland
      • Department of Physics
      Auckland, Auckland, New Zealand