[Show abstract][Hide abstract] ABSTRACT: We show that the particle number distribution of diamond modes, modes that
are localised in a finite space-time 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.
[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 Einstein-Podolsky-Rosen (EPR) state of light, the workhorse of
continuous-variable 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
bench-marking step towards the realization of a practical continuous-variable
quantum repeater and other CV quantum protocols.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
New Journal of Physics 02/2015; 17(6). DOI:10.1088/1367-2630/17/6/063037 · 3.67 Impact Factor
[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 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.
[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 single-photon 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.
International Journal of Quantum Information 11/2014; 12(07n08):1560015. DOI:10.1142/S0219749915600151 · 0.99 Impact Factor
[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 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
[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.
Physical Review D 10/2014; 90(8). DOI:10.1103/PhysRevD.90.084022 · 4.86 Impact Factor
[Show abstract][Hide abstract] 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.
[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
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
New Journal of Physics 09/2014; 17(6). DOI:10.1088/1367-2630/17/6/063008 · 3.67 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[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 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[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 one-sided device
independent schemes are experimentally achievable with existing technology for
transmission over realistic channels.