T. C. Ralph

University of Queensland, Brisbane, Queensland, Australia

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Publications (346)1275.48 Total impact

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    ABSTRACT: We propose a Bell measurement scheme by employing a logical qubit in Greenberger-Horne-Zeilinger (GHZ) entanglement with an arbitrary number of photons. Remarkably, the success probability of the Bell measurement as well as teleportation of the GHZ entanglement can be made arbitrarily high using only linear optics elements and photon on-off measurements as the number of photons increases. Our scheme outperforms previous proposals using single photon qubits when comparing the success probabilities in terms of the average photon usages. It has another important advantage for experimental feasibility that it does not require photon number resolving measurements. Our proposal provides an alternative candidate for all-optical quantum information processing.
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    ABSTRACT: Although the strengths of optical non-linearities available experimentally have been rapidly increasing in recent years, significant challenges remain to using such non-linearities to produce useful quantum devices such as efficient optical Bell state analysers or universal quantum optical gates. Here we describe a new approach that avoids the current limitations by combining strong non-linearities with active Gaussian operations in efficient protocols for Bell state analysers and Controlled-Sign gates.
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    ABSTRACT: We introduce an operational discord-type measure for quantifying nonclassical correlations in bipartite Gaussian states based on using Gaussian measurements. We refer to this measure as operational Gaussian discord (OGD). It is defined as the difference between the entropies of two conditional probability distributions associated to one subsystem, which are obtained by performing optimal local and joint Gaussian measurements. We demonstrate the operational significance of this measure in terms of a Gaussian quantum protocol for extracting maximal information about an encoded classical signal. As examples, we calculate OGD for several Gaussian states in the standard form.
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    ABSTRACT: In general relativity, closed timelike curves can break causality with remarkable and unsettling consequences. At the classical level, they induce causal paradoxes disturbing enough to motivate conjectures that explicitly prevent their existence. At the quantum level, resolving such paradoxes induce radical benefits - from cloning unknown quantum states to solving problems intractable to quantum computers. Instinctively, one expects these benefits to vanish if causality is respected. Here we show that in harnessing entanglement, we can efficiently solve NP-complete problems and clone arbitrary quantum states - even when all time-travelling systems are completely isolated from the past. Thus, the many defining benefits of closed timelike curves can still be harnessed, even when causality is preserved. Our results unveil the subtle interplay between entanglement and general relativity, and significantly improve the potential of probing the radical effects that may exist at the interface between relativity and quantum theory.
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    ABSTRACT: We study the operational regime of a noiseless linear amplifier (NLA) based on quantum scissors that can nondeterministically amplify the one photon component of a quantum state with weak excitation. It has been shown that an arbitrarily large quantum state can be amplified by first splitting it into weak excitation states using a network of beamsplitters. The output states of the network can then be coherently recombined. In this paper, we analyse the performance of such a device for distilling entanglement after transmission through a lossy quantum channel, and look at two measures to determine the efficacy of the NLA. The measures used are the amount of entanglement achievable and the final purity of the output amplified entangled state. We study the performances of both a single and a two-element NLA for amplifying weakly excited states. Practically, we show that it may be advantageous to work with a limited number of stages.
    Journal of Physics B Atomic Molecular and Optical Physics 10/2014; 47(21):215503. DOI:10.1088/0953-4075/47/21/215503 · 1.92 Impact Factor
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    ABSTRACT: Entanglement distillation is an indispensable ingredient in extended quantum communication networks. Distillation protocols are necessarily non-deterministic and require advanced experimental techniques such as noiseless amplification. Recently it was shown that the benefits of noiseless amplification could be extracted by performing a post-selective filtering of the measurement record to improve the performance of quantum key distribution. We apply this protocol to entanglement degraded by transmission loss of up to the equivalent of 100km of optical fibre. We measure an effective entangled resource stronger than that achievable by even a maximally entangled resource passively transmitted through the same channel. We also provide a proof-of-principle demonstration of secret key extraction from an otherwise insecure regime. The measurement-based noiseless linear amplifier offers two advantages over its physical counterpart: ease of implementation and near optimal probability of success. It should provide an effective and versatile tool for a broad class of entanglement-based quantum communication protocols.
    Nature Photonics 09/2014; 8(4). DOI:10.1038/nphoton.2014.49 · 29.96 Impact Factor
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    T. C. Ralph, N. Walk
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    ABSTRACT: Quantum Key Distribution is a quantum communication technique in which random numbers are encoded on quantum systems, usually photons, and sent from one party, Alice, to another, Bob. Using the data sent via the quantum signals, supplemented by classical communication, it is possible for Alice and Bob to share an unconditionally secure secret key. This is not possible if only classical signals are sent. Whilst this last statement is a long standing result from quantum information theory it turns out only to be true in a non-relativistic setting. If relativistic quantum field theory is considered we show it is possible to distribute an unconditionally secure secret key without sending a quantum signal, instead harnessing the intrinsic entanglement between different regions of space time. The protocol is practical in free space given horizon technology and might be testable in principle in the near term using microwave technology.
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    ABSTRACT: We pose a randomized 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 a Gaussian input state, a linear optical network, and nonadaptive photon counting measurements. All the elements required to build such a processor currently exist. The demonstration of such a device would provide empirical evidence that quantum computers can, indeed, outperform classical computers and could lead to applications.
    Physical Review Letters 09/2014; 113(10):100502. · 7.73 Impact Factor
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    ABSTRACT: We propose a quantum experiment to measure with high precision the Schwarzschild space-time parameters of the Earth. The scheme can also be applied to measure distances by taking into account the curvature of the Earth's space-time. As a wave-packet of (entangled) light is sent from the Earth to a satellite it is red-shifted and deformed due to the curvature of space-time. Measurements after the propagation enable the estimation of the space-time parameters. We compare our results with the state of the art, which involves classical measurement methods, and discuss what developments are required in space-based quantum experiments to improve on the current measurement of the Schwarzschild radius of the Earth.
    Physical Review D 08/2014; 90(12). DOI:10.1103/PhysRevD.90.124001 · 4.86 Impact Factor
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    ABSTRACT: We present a protocol based on continuous-variable quantum teleportation and Gaussian post- selection that can be used to correct errors introduced by a lossy channel. We first show that the global transformation enacted by the protocol is equivalent to an effective system composed of a noiseless amplification (or attenuation), and an effective quantum channel, which can in theory have no loss and an amount of thermal noise arbitrarily small, hence tending to an identity channel. An application of our protocol is the probabilistic purification of quantum non-Gaussian states using only Gaussian operations.
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    ABSTRACT: Considering the problem of sampling from the output photon-counting probability distribution of a linear-optical network for input Gaussian states, we obtain new results that are of interest from both quantum theory and the complexity theory point of view. We derive a general formula for calculating the output probabilities. By considering input thermal states, we show that the output probabilities are proportional to permanents of positive definite Hermitian matrices. It is believed that approximating permanents of complex matrices in general is a #P-hard problem. However, we show that these permanents can be approximated with an algorithm within the third level of the polynomial hierarchy, as there exists an efficient classical algorithm for sampling from the output probability distribution. On the other hand, considering input squeezed-vacuum states, we show the output probabilities are proportional to a quantity which is, for at least a specific configuration, #P-hard to approximate.
    Physical Review Letters 08/2014; 114(6). DOI:10.1103/PhysRevLett.114.060501 · 7.73 Impact Factor
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    ABSTRACT: Closed timelike curves are among the most controversial features of modern physics. As legitimate solutions to Einstein's field equations, they allow for time travel, which instinctively seems paradoxical. However, in the quantum regime these paradoxes can be resolved, leaving closed timelike curves consistent with relativity. The study of these systems therefore provides valuable insight into nonlinearities and the emergence of causal structures in quantum mechanics-essential for any formulation of a quantum theory of gravity. Here we experimentally simulate the nonlinear behaviour of a qubit interacting unitarily with an older version of itself, addressing some of the fascinating effects that arise in systems traversing a closed timelike curve. These include perfect discrimination of non-orthogonal states and, most intriguingly, the ability to distinguish nominally equivalent ways of preparing pure quantum states. Finally, we examine the dependence of these effects on the initial qubit state, the form of the unitary interaction and the influence of decoherence.
    Nature Communications 06/2014; 5:4145. DOI:10.1038/ncomms5145 · 10.74 Impact Factor
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    T. C. Ralph, J. Pienaar
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    ABSTRACT: The event formalism is a non-linear extension of quantum field theory designed to be compatible with the closed time-like curves that appear in general relativity. Whilst reducing to standard quantum field theory in flat space-time the formalism leads to testably different predictions for entanglement distribution in curved space. In this paper we introduce a more general version of the formalism and use it to analyse the practicality of an experimental test of its predictions in the earth's gravitational well.
    New Journal of Physics 06/2014; 16(8). DOI:10.1088/1367-2630/16/8/085008 · 3.67 Impact Factor
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    ABSTRACT: We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and certain class of non-Gaussian states. We also demonstrate that discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interaction.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: We simultaneously generate photon-subtracted squeezed vacuum and squeezed vacuum at three frequencies from an optical parametric oscillator by utilizing its frequency nondegenerate side-bands. Quantum non-Gaussianity is demonstrated by applying a novel character witness.
    CLEO: QELS_Fundamental Science; 06/2014
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    Nathan Walk, Howard M Wiseman, Timothy C Ralph
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    ABSTRACT: By extending recently developed entropic uncertainty relations in the continuous variable regime we derive bounds upon the secret key rate of Gaussian modulated continuous variable quantum key distribution protocols in the limit of long key length. For several protocols the bounds obtained in this manner can be shown to be one sided device independent, including a protocol that uses only coherent states. Though the derived uncertainty relation is not tight, and neither are the subsequent key rates, we find that one-sided device independent schemes are experimentally achievable with existing technology for transmission over realistic channels.
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    ABSTRACT: We discuss how to experimentally detect a recently proposed measure to quantify macroscopic quantum superpositions [Phys. Rev. Lett. 106, 220401 (2011)], namely, "macroscopic quantumness" $\mathcal{I}$. After reviewing distinguished properties of the measure $\mathcal{I}$, we point out that it can be detected without full tomography of the density matrix using an overlap measurement together with added decoherence. We review schemes for overlap measurements of harmonic oscillators and two-level systems based on swap operations, and discuss how to apply them to our purpose. An overlap measurement scheme based on controlled-swap operations is introduced for single-photon polarization qubits. In order to discuss experimental limitations, we analyze the effects of coarse-graining and detection inefficiency.
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    ABSTRACT: We investigate how to experimentally detect a recently proposed measure to quantify macroscopic quantum superpositions [Phys. Rev. Lett. 106, 220401 (2011)], namely, “macroscopic quantumness” I. Schemes based on overlap measurements for harmonic oscillator states and for qubit states are extensively investigated. Effects of detection inefficiency and coarse-graining are analyzed in order to assess feasibility of the schemes.
    Journal of the Optical Society of America B 03/2014; 31(12). DOI:10.1364/JOSAB.31.003057 · 1.81 Impact Factor
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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.
    Physical Review Letters 11/2013; 113(6). DOI:10.1103/PhysRevLett.113.063601 · 7.73 Impact Factor
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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; DOI:10.1103/PhysRevA.88.052335 · 2.99 Impact Factor

Publication Stats

8k Citations
1,275.48 Total Impact Points

Institutions

  • 1970–2015
    • University of Queensland
      • • School of Mathematics and Physics
      • • Faculty of Science
      Brisbane, Queensland, Australia
  • 2013
    • University of Bristol
      • Department of Electrical and Electronic Engineering
      Bristol, England, United Kingdom
  • 2010
    • Max-Planck-Institut für die Physik des Lichts
      • Max Planck Institute for the Science of Light
      Erlangen, Bavaria, Germany
  • 2007–2008
    • University of New South Wales
      • • Centre for Quantum Computer Technology (CQCT)
      • • School of Engineering and Information Technology
      Kensington, New South Wales, Australia
  • 1991–2006
    • Australian National University
      • • Department of Quantum Science (DQS)
      • • Department of Theoretical Physics
      Canberra, Australian Capital Territory, Australia
  • 2005
    • University of Canberra
      Canberra, Australian Capital Territory, Australia
  • 1997
    • University of Auckland
      • Department of Physics
      Auckland, Auckland, New Zealand