344 research outputs found
Mutually unbiased measurements for high-dimensional time-bin based photonic states
The task of measuring in two mutually unbiased bases is central to many
quantum information protocols, as well as being of fundamental interest.
Increasingly, there is an experimental focus on generating and controlling
high-dimensional photonic states. One approach is to use the arrival time of a
photon, which can be split into discrete time bins. An important problem
associated with such states is the difficulty in experimentally realizing a
measurement that is mutually unbiased with respect to the time-of-arrival. We
propose a simple and compact scheme to measure in both the time of arrival
basis and a basis that is approximately mutually unbiased with respect to the
arrival time.Comment: Accepted in EPL, 4.1 pages and 2 figure
Information communicated by entangled photon pairs
A key goal of quantum communication is to determine the maximum number of
bits shared between two quantum systems. An important example of this is in
entanglement based quantum key distribution (QKD) schemes. A realistic
treatment of this general communication problem must take account of the
nonideal nature of the entanglement source and the detectors. The aim of this
paper is to give such a treatment. We obtain analytic expression for the mutual
information in terms of experimental parameters. The results are applied to
communication schemes that rely on spontaneous parametric down conversion to
generate entangled photons. We show that our results can be applied to tasks
such as calculating the optimal rate of bits per photon in high dimensional
time bin encoded QKD protocols (prior to privacy amplification). A key finding
for such protocols is that by using realistic experimental parameters, one can
obtain over 10 bits per photon. We also show how our results can be applied to
characterize the capacity of a fibre array and to quantify entanglement using
mutual information.Comment: 9 pages, 5 figures, accepted for publication in Phys Rev
Perfect transfer of multiple excitations in quantum networks
We present a general formalism to the problem of perfect state-transfer
(PST), where the state involves multiple excitations of the quantum network. A
key feature of our formalism is that it allows for inclusion of nontrivial
interactions between the excitations. Hence, it is perfectly suited to
addressing the problem of PST in the context of various types of physical
realizations. The general formalism is also flexible enough to account for
situations where multiple excitations are "focused" onto the same site.Comment: close to the version published in Phys. Rev. A. In version 2, a typo
has been corrected in Sec. III
Cavity-enabled high-dimensional quantum key distribution
High-dimensional quantum key distribution (QKD) offers the possibility of encoding multiple bits of key on a single entangled photon pair. An experimentally promising approach to realizing this is to use energy–time entanglement. Currently, however, the control of very high-dimensional entangled photons is challenging. We present a simple and experimentally compact approach, which is based on a cavity that allows one to measure two different bases: the time of arrival and another that is approximately mutually unbiased to the arrival time. We quantify the errors in the setup, due both to the approximate nature of the mutually unbiased measurement and as a result of experimental errors. It is shown that the protocol can be adapted using a cut-off so that it is robust against the considered errors, even within the regime of up to 10 bits per photon pair
Recovering full coherence in a qubit by measuring half of its environment
When quantum systems interact with the environment they lose their quantum
properties, such as coherence. Quantum erasure makes it possible to restore
coherence in a system by measuring its environment, but accessing the whole of
it may be prohibitive: realistically one might have to concentrate only on an
accessible subspace and neglect the rest. If that is the case, how good is
quantum erasure? In this work we compute the largest coherence that we can expect to recover in a qubit, as a function of
the dimension of the accessible and of the inaccessible subspaces of its
environment. We then imagine the following game: we are given a uniformly
random pure state of qubits and we are asked to compute the largest
coherence that we can retrieve on one of them by optimally measuring a certain
number of the others. We find a surprising effect around the
value : the recoverable coherence sharply transitions between 0
and 1, indicating that in order to restore full coherence on a qubit we need
access to only half of its physical environment (or in terms of degrees of
freedom to just the square root of them). Moreover, we find that the
recoverable coherence becomes a typical property of the whole ensemble as
grows.Comment: 4 pages, 5 figure
The information of high-dimensional time-bin encoded photons
We determine the shared information that can be extracted from time-bin
entangled photons using frame encoding. We consider photons generated by a
general down-conversion source and also model losses, dark counts and the
effects of multiple photons within each frame. Furthermore, we describe a
procedure for including other imperfections such as after-pulsing, detector
dead-times and jitter. The results are illustrated by deriving analytic
expressions for the maximum information that can be extracted from
high-dimensional time-bin entangled photons generated by a spontaneous
parametric down conversion. A key finding is that under realistic conditions
and using standard SPAD detectors one can still choose frame size so as to
extract over 10 bits per photon. These results are thus useful for experiments
on high-dimensional quantum-key distribution system.Comment: 18 pages, 6 figure
Communication in quantum networks of logical bus topology
Perfect state transfer (PST) is discussed in the context of passive quantum
networks with logical bus topology, where many logical nodes communicate using
the same shared media, without any external control. The conditions under
which, a number of point-to-point PST links may serve as building blocks for
the design of such multi-node networks are investigated. The implications of
our results are discussed in the context of various Hamiltonians that act on
the entire network, and are capable of providing PST between the logical nodes
of a prescribed set in a deterministic manner.Comment: 9 pages, 1 figur
Security of high-dimensional quantum key distribution protocols using Franson interferometers
Franson interferometers are increasingly being proposed as a means of
securing high-dimensional energy-time entanglement-based quantum key
distribution (QKD) systems. Heuristic arguments have been proposed that purport
to demonstrate the security of these schemes. We show, however, that such
systems are vulnerable to attacks that localize the photons to several
temporally separate locations. This demonstrates that a single pair of Franson
interferometers is not a practical approach to securing high-dimensional
energy-time entanglement based QKD. This observations leads us to investigate
the security of modified Franson-based-protocols, where Alice and Bob have two
or more Franson interferometers. We show that such setups can improve the
sensitivity against attacks that localize the photons to multiple temporal
locations. While our results do not constituting a full security proof, they do
show that a single pair of Franson interferometers is not secure and that
multiple such interferometers could be a promising candidate for experimentally
realizable high-dimensional QKD.Comment: 14 pages (single column format
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