Publications (341)1253.16 Total impact
 [Show abstract] [Hide abstract]
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 NPcomplete problems and clone arbitrary quantum states  even when all timetravelling 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.12/2014;  [Show abstract] [Hide abstract]
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 twoelement 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. · 1.92 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Entanglement distillation is an indispensable ingredient in extended quantum communication networks. Distillation protocols are necessarily nondeterministic 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 postselective 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 proofofprinciple demonstration of secret key extraction from an otherwise insecure regime. The measurementbased 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 entanglementbased quantum communication protocols.Nature Photonics 09/2014; 8(4). · 29.96 Impact Factor  [Show abstract] [Hide abstract]
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 nonrelativistic 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.09/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We pose a randomized bosonsampling 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 
Article: Channel purification via continuousvariable quantum teleportation with Gaussian postselection
[Show abstract] [Hide abstract]
ABSTRACT: We present a protocol based on continuousvariable 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 nonGaussian states using only Gaussian operations.08/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Considering the problem of sampling from the output photoncounting probability distribution of a linearoptical 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 #Phard 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 squeezedvacuum states, we show the output probabilities are proportional to a quantity which is, for at least a specific configuration, #Phard to approximate.08/2014;  [Show abstract] [Hide abstract]
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 mechanicsessential 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 nonorthogonal 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. · 10.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The event formalism is a nonlinear extension of quantum field theory designed to be compatible with the closed timelike curves that appear in general relativity. Whilst reducing to standard quantum field theory in flat spacetime 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). · 3.67 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We simultaneously generate photonsubtracted squeezed vacuum and squeezed vacuum at three frequencies from an optical parametric oscillator by utilizing its frequency nondegenerate sidebands. Quantum nonGaussianity is demonstrated by applying a novel character witness.CLEO: QELS_Fundamental Science; 06/2014 
Conference Paper: Experimental Verification of Quantum Discord and Operational Significance of Discord Consumption
[Show abstract] [Hide abstract]
ABSTRACT: We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and certain class of nonGaussian 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  [Show abstract] [Hide abstract]
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 onesided device independent schemes are experimentally achievable with existing technology for transmission over realistic channels.05/2014;  [Show abstract] [Hide abstract]
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 twolevel systems based on swap operations, and discuss how to apply them to our purpose. An overlap measurement scheme based on controlledswap operations is introduced for singlephoton polarization qubits. In order to discuss experimental limitations, we analyze the effects of coarsegraining and detection inefficiency.04/2014;  [Show abstract] [Hide abstract]
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 coarsegraining are analyzed in order to assess feasibility of the schemes.Journal of the Optical Society of America B 03/2014; 31(12). · 1.81 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that a set of optical memories can act as a reconfigurable linear optical network operating on frequencymultiplexed optical states. Our protocol is applicable to any quantum memories that employ offresonant 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 Nmode 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).  [Show abstract] [Hide abstract]
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; · 2.99 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and a certain class of nonGaussian 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 nonGaussian 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 nonGaussian states.Journal of Physics B Atomic Molecular and Optical Physics 10/2013; 47(2). · 1.92 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Entanglement between quantum and classical objects is of special interest in the context of fundamental studies of quantum mechanics and potential applications to quantum information processing. In quantum optics, single photons are treated as light quanta while coherent states are considered the most classical among all pure states. Recently, entanglement between a single photon and a coherent state in a freetraveling field was identified to be a useful resource for optical quantum information processing. However, it was pointed out to be extremely difficult to generate such states since it requires a clean crossKerr nonlinear interaction. Here, we devise and experimentally demonstrate a scheme to generate such hybrid entanglement by implementing a coherent superposition of two distinct quantum operations. The generated states clearly show entanglement between the two different types of states. Our work opens a way to generate hybrid entanglement of a larger size and to develop efficient quantum information processing using such a new type of qubits.Nature Photonics 09/2013; 8(7). · 29.96 Impact Factor  Nature 08/2013; 500(7462):2823. · 42.35 Impact Factor

Article: Highfidelity teleportation of continuousvariable quantum States using delocalized single photons.
[Show abstract] [Hide abstract]
ABSTRACT: Traditional continuousvariable teleportation can only approach unit fidelity in the limit of an infinite (and unphysical) amount of squeezing. We describe a new method for continuousvariable teleportation that approaches unit fidelity with finite resources. The protocol is not based on squeezed states as in traditional teleportation but on an ensemble of single photon entangled states. We characterize the teleportation scheme with coherent states, mesoscopic superposition states, and twomode squeezed states and we find several situations in which nearunity teleportation fidelity can be obtained with modest resources.Physical Review Letters 08/2013; 111(5):050504. · 7.73 Impact Factor
Publication Stats
7k  Citations  
1,253.16  Total Impact Points  
Top Journals
Institutions

2000–2014

University of Queensland
 School of Mathematics and Physics
Brisbane, Queensland, Australia


2013

Technical University of Denmark
 Department of Physics
Copenhagen, Capital Region, Denmark


2011

University of Bristol
 Department of Electrical and Electronic Engineering
Bristol, ENG, United Kingdom


2010

MaxPlanckInstitut für die Physik des Lichts
Erlangen, Bavaria, Germany 
Perimeter Institute for Theoretical Physics
Waterloo, Ontario, Canada


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

FriedrichAlexander Universität ErlangenNürnberg
 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
