276 research outputs found
Quantum discord bounds the amount of distributed entanglement
The ability to distribute quantum entanglement is a prerequisite for many
fundamental tests of quantum theory and numerous quantum information protocols.
Two distant parties can increase the amount of entanglement between them by
means of quantum communication encoded in a carrier that is sent from one party
to the other. Intriguingly, entanglement can be increased even when the
exchanged carrier is not entangled with the parties. However, in light of the
defining property of entanglement stating that it cannot increase under
classical communication, the carrier must be quantum. Here we show that, in
general, the increase of relative entropy of entanglement between two remote
parties is bounded by the amount of non-classical correlations of the carrier
with the parties as quantified by the relative entropy of discord. We study
implications of this bound, provide new examples of entanglement distribution
via unentangled states and put further limits on this phenomenon.Comment: 8 pages, 1 figure, RevTeX4; Accepted for publication in Phys. Rev.
Let
Quantum Correlations in Multipartite Quantum Systems
We review some concepts and properties of quantum correlations, in particular
multipartite measures, geometric measures and monogamy relations. We also
discuss the relation between classical and total correlationsComment: to be published as a chapter of the book "Lectures on general quantum
correlations and their applications" edited by F. Fanchini, D. Soares-Pinto,
and G. Adesso (Springer, 2017
Low-dimensional quite noisy bound entanglement with cryptographic key
We provide a class of bound entangled states that have positive distillable
secure key rate. The smallest state of this kind is 4 \bigotimes 4. Our class
is a generalization of the class presented in [1] (IEEE Trans. Inf. Theory 54,
2621 (2008); arXiv:quant-ph/0506203). It is much wider, containing, in
particular, states from the boundary of PPT entangled states (all of the states
in the class in [1] were of this kind) but also states inside the set of PPT
entangled states, even, approaching the separable states. This generalization
comes with a price: for the wider class a positive key rate requires, in
general, apart from the one-way Devetak-Winter protocol (used in [1]) also the
recurrence preprocessing and thus effectively is a two-way protocol. We also
analyze the amount of noise that can be admixtured to the states of our class
without losing key distillability property which may be crucial for
experimental realization. The wider class contains key-distillable states with
higher entropy (up to 3.524, as opposed to 2.564 for the class in [1]).Comment: 10 pages, final version for J. Phys. A: Math. Theo
Entanglement distribution and quantum discord
Establishing entanglement between distant parties is one of the most
important problems of quantum technology, since long-distance entanglement is
an essential part of such fundamental tasks as quantum cryptography or quantum
teleportation. In this lecture we review basic properties of entanglement and
quantum discord, and discuss recent results on entanglement distribution and
the role of quantum discord therein. We also review entanglement distribution
with separable states, and discuss important problems which still remain open.
One such open problem is a possible advantage of indirect entanglement
distribution, when compared to direct distribution protocols.Comment: 7 pages, 2 figures, contribution to "Lectures on general quantum
correlations and their applications", edited by Felipe Fanchini, Diogo
Soares-Pinto, and Gerardo Adess
Experimental investigation of classical and quantum correlations under decoherence
It is well known that many operations in quantum information processing
depend largely on a special kind of quantum correlation, that is, entanglement.
However, there are also quantum tasks that display the quantum advantage
without entanglement. Distinguishing classical and quantum correlations in
quantum systems is therefore of both fundamental and practical importance. In
consideration of the unavoidable interaction between correlated systems and the
environment, understanding the dynamics of correlations would stimulate great
interest. In this study, we investigate the dynamics of different kinds of
bipartite correlations in an all-optical experimental setup. The sudden change
in behaviour in the decay rates of correlations and their immunity against
certain decoherences are shown. Moreover, quantum correlation is observed to be
larger than classical correlation, which disproves the early conjecture that
classical correlation is always greater than quantum correlation. Our
observations may be important for quantum information processing.Comment: 7 pages, 4 figures, to appear in Nature Communication
Dynamics of multipartite quantum correlations under decoherence
Quantum discord is an optimal resource for the quantification of classical
and non-classical correlations as compared to other related measures. Geometric
measure of quantum discord is another measure of quantum correlations.
Recently, the geometric quantum discord for multipartite states has been
introduced by Jianwei Xu [arxiv:quant/ph.1205.0330]. Motivated from the recent
study [Ann. Phys. 327 (2012) 851] for the bipartite systems, I have
investigated global quantum discord (QD) and geometric quantum discord (GQD)
under the influence of external environments for different multipartite states.
Werner-GHZ type three-qubit and six-qubit states are considered in inertial and
non-inertial settings. The dynamics of QD and GQD is investigated under
amplitude damping, phase damping, depolarizing and flipping channels. It is
seen that the quantum discord vanishes for p>0.75 in case of three-qubit GHZ
states and for p>0.5 for six qubit GHZ states. This implies that multipartite
states are more fragile to decoherence for higher values of N. Surprisingly, a
rapid sudden death of discord occurs in case of phase flip channel. However,
for bit flip channel, no sudden death happens for the six-qubit states. On the
other hand, depolarizing channel heavily influences the QD and GQD as compared
to the amplitude damping channel. It means that the depolarizing channel has
the most destructive influence on the discords for multipartite states. From
the perspective of accelerated observers, it is seen that effect of environment
on QD and GQD is much stronger than that of the acceleration of non-inertial
frames. The degradation of QD and GQD happens due to Unruh effect. Furthermore,
QD exhibits more robustness than GQD when the multipartite systems are exposed
to environment.Comment: 15 pages, 4 figures, 4 table
Quantum Correlation in One-dimensional Extend Quantum Compass Model
We study the correlations in the one-dimensional extended quantum compass
model in a transverse magnetic field. By exactly solving the Hamiltonian, we
find that the quantum correlation of the ground state of one-dimensional
quantum compass model is vanishing. We show that quantum discord can not only
locate the quantum critical points, but also discern the orders of phase
transitions. Furthermore, entanglement quantified by concurrence is also
compared.Comment: 8 pages, 14 figures, to appear in Eur. Phys. J.
A Generalization of Quantum Stein's Lemma
We present a generalization of quantum Stein's Lemma to the situation in
which the alternative hypothesis is formed by a family of states, which can
moreover be non-i.i.d.. We consider sets of states which satisfy a few natural
properties, the most important being the closedness under permutations of the
copies. We then determine the error rate function in a very similar fashion to
quantum Stein's Lemma, in terms of the quantum relative entropy.
Our result has two applications to entanglement theory. First it gives an
operational meaning to an entanglement measure known as regularized relative
entropy of entanglement. Second, it shows that this measure is faithful, being
strictly positive on every entangled state. This implies, in particular, that
whenever a multipartite state can be asymptotically converted into another
entangled state by local operations and classical communication, the rate of
conversion must be non-zero. Therefore, the operational definition of
multipartite entanglement is equivalent to its mathematical definition.Comment: 30 pages. (see posting by M. Piani arXiv:0904.2705 for a different
proof of the strict positiveness of the regularized relative entropy of
entanglement on every entangled state). published version
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