Publications (361)1297.15 Total impact
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ABSTRACT: We provide general sufficient conditions for the efficient classical simulation of quantumoptics experiments that involve inputting states to a quantum process and making measurements at the output. The first condition is based on the negativity of phasespace quasiprobability distributions (PQDs) of the output state of the process and the output measurements; the second one is based on the negativity of PQDs of the input states, the output measurements, and the transition function associated with the process. We apply these conditions to implementations of boson sampling that use singlephoton or spontaneousparametricdownconversion sources. We show that above some threshold for loss and noise, the bosonsampling experiments are classically simulatable.  [Show abstract] [Hide abstract]
ABSTRACT: We present a detailed analysis of the Bell measurement scheme proposed in [Phys. Rev. Lett. 114, 113603 (2015)] based on a logical qubit using GreenbergerHorneZeilinger (GHZ) entanglement of photons. The success probability of the proposed Bell measurement can be made arbitrarily high using only linear optics as the number of photons in a logical qubit increases. We compare our scheme with all the other proposals, using singlephoton qubits, coherentstate qubits or hybrid qubits, suggested to enhance the efficiency of the Bell measurement. As a remarkable advantage, our scheme requires only photon onoff measurements, while photon number resolving detectors are necessary for all the other proposals. We find that the scheme based on coherentstate qubits shows the best performance with respect to the attained success probability in terms of the average number of photons used in the process, while our scheme outperforms the schemes using singlephoton qubits. We finally show that efficient quantum communication and faulttolerant quantum computation can be realized using our approach. 
Article: Spacetime diamonds
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ABSTRACT: We show that the particle number distribution of diamond modes, modes that are localised in a finite spacetime region, are thermal for the Minkowski vacuum state of a massless scalar field, an analogue to the Unruh effect. The temperature of the diamond is inversely proportional to its size. An inertial observer can detect this thermal radiation by coupling to the diamond modes using an appropriate energy scaled detector. We further investigate the correlations between various diamonds and find that entanglement between adjacent diamonds dominates. 
Article: Properties of hybrid entanglement between discrete and continuousvariable states of light
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ABSTRACT: We discuss some of the main features of a recentlygenerated form of hybrid entanglement between discrete and continuousvariable states of light. Ideally, such a kind of entanglement should involve singlephoton and coherent states as key representatives of the respective categories of states. Here we investigate the characteristics and limits of a scheme that, relying on a superposition of photoncreation operators onto two distinct modes, realizes the above ideal form of hybrid entanglement in an approximate way.  [Show abstract] [Hide abstract]
ABSTRACT: Entanglement distillation is a process via which the strength and purity of quantum entanglement can be increased probabilistically. It is a key step in many quantum communication and computation protocols. In particular, entanglement distillation is a necessary component of the quantum repeater, a device which counters the degradation of entanglement that inevitably occurs due to losses in a communication line. Here we report an experiment on distilling the EinsteinPodolskyRosen (EPR) state of light, the workhorse of continuousvariable entanglement, using the technique of noiseless amplification. In contrast to previous implementations, the entanglement enhancement factor achievable by our technique is not fundamentally limited and permits recovering an EPR state with a macroscopic level of entanglement no matter how low the initial entanglement or how high the loss may be. In particular, we recover the original level of entanglement after one of the EPR modes has passed through a channel with a loss factor of 20. The level of entanglement in our distilled state is higher than that achievable by direct transmission of any state through a similar loss channel. This is a key benchmarking step towards the realization of a practical continuousvariable quantum repeater and other CV quantum protocols.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a Bell measurement scheme by employing a logical qubit in GreenbergerHorneZeilinger (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 onoff 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 alloptical quantum information processing.  [Show abstract] [Hide abstract]
ABSTRACT: Although the strengths of optical nonlinearities available experimentally have been rapidly increasing in recent years, significant challenges remain to using such nonlinearities 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 nonlinearities with active Gaussian operations in efficient protocols for Bell state analysers and ControlledSign gates.  [Show abstract] [Hide abstract]
ABSTRACT: We introduce an operational discordtype 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.  [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.  [Show abstract] [Hide abstract]
ABSTRACT: The realization of hybrid entanglement between a microscopic (quantum) and a macroscopic (classical) system, in analogy to the situation of the famous Schrödinger's cat paradox, is an important milestone, both from the fundamental perspective and for possible applications in the processing of quantum information. The most straightforward optical implementation of this condition is that of the entanglement between a singlephoton and a coherent state. In this work, we describe the first step towards the generation of this type of hybrid entanglement from the experimental perspective.  [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.  [Show abstract] [Hide abstract]
ABSTRACT: We report a linear optical Fredkin gate using an entanglement resource and an expanded Hilbert space. Additionally we demonstrate verification of weak entanglement which does not require trust in the measurement devices or their operators.  [Show abstract] [Hide abstract]
ABSTRACT: Based on homodyne detection, we discuss how the presence of an event horizon affects quantum communication between an inertial partner, Alice, and a uniformly accelerated partner, Rob. We show that there exists a low frequency cutoff for Rob's homodyne detector that maximizes the signal to noise ratio and it approximately corresponds to the Unruh frequency. In addition, the low frequency cutoff which minimizes the conditional variance between Alice's input state and Rob's output state is also approximately equal to the Unruh frequency. Thus the Unruh frequency provides a natural low frequency cutoff in order to optimize quantum communication of both classical and quantum information between Alice and Rob.  [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.  [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. 
Article: Boson sampling from a Gaussian state
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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.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a quantum experiment to measure with high precision the Schwarzschild spacetime parameters of the Earth. The scheme can also be applied to measure distances by taking into account the curvature of the Earth's spacetime. As a wavepacket of (entangled) light is sent from the Earth to a satellite it is redshifted and deformed due to the curvature of spacetime. Measurements after the propagation enable the estimation of the spacetime parameters. We compare our results with the state of the art, which involves classical measurement methods, and discuss what developments are required in spacebased quantum experiments to improve on the current measurement of the Schwarzschild radius of the Earth. 
Article: Channel purification via continuousvariable quantum teleportation with Gaussian postselection
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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.  [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.  [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.
Publication Stats
10k  Citations  
1,297.15  Total Impact Points  
Top Journals
Institutions

2015

University of New Mexico
Albuquerque, New Mexico, United States


19702015

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


2011

The University of York
 Department of Computer Science
York, England, United Kingdom


2010

MaxPlanckInstitut für die Physik des Lichts
 Max Planck Institute for the Science of Light
Erlangen, Bavaria, Germany


2008

University of New South Wales
 Centre for Quantum Computer Technology (CQCT)
Kensington, New South Wales, Australia


19912004

Australian National University
 • Department of Quantum Science (DQS)
 • Department of Theoretical Physics
Canberra, Australian Capital Territory, Australia


1997

University of Auckland
 Department of Physics
Auckland, Auckland, New Zealand
