56 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.
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Reference frames for Bell inequality violation in the presence of superselection rules
Superselection rules (SSRs) constrain the allowed states and operations in quantum theory. They limit preparations and measurements and hence impact our ability to observe non-locality, in particular the violation of Bell inequalities. We show that a reference frame compatible with a particle number SSR does not allow observers to violate a Bell inequality if and only if it is prepared using only local operations and classical communication. In particular, jointly prepared separable reference frames are sufficient for obtaining violations of a Bell inequality. We study the size and non-local properties of such reference frames using superselection-induced variance. These results suggest the need for experimental Bell tests in the presence of superselection
Invariant quantum discord in qubit-qutrit systems under local dephasing
We investigate the dynamics of quantum discord and entanglement for a
class of mixed qubit-qutrit states assuming that only the qutrit is under the action of a dephasing channel. We demonstrate that even though the entanglement in the qubit-qutrit state disappears in a finite time interval, partial coherence left in the system enables quantum discord to remain invariant throughout the whole time evolution
Quantum correlations in a few-atom spin-1 Bose-Hubbard model
We study the thermal quantum correlations and entanglement in spin-1 Bose-Hubbard model with two and three particles. While we use negativity to calculate entanglement, more general non-classical correlations are quantified using a new measure based on a necessary and sufficient condition for zero-discord state. We demonstrate that the energy level crossings in the ground state of the system are signalled by both the behavior of thermal quantum correlations and entanglement
Decoherence on a two-dimensional quantum walk using four- and two-state particle
We study the decoherence effects originating from state flipping and
depolarization for two-dimensional discrete-time quantum walks using four-state
and two-state particles. By quantifying the quantum correlations between the
particle and position degree of freedom and between the two spatial ()
degrees of freedom using measurement induced disturbance (MID), we show that
the two schemes using a two-state particle are more robust against decoherence
than the Grover walk, which uses a four-state particle. We also show that the
symmetries which hold for two-state quantum walks breakdown for the Grover
walk, adding to the various other advantages of using two-state particles over
four-state particles.Comment: 12 pages, 16 figures, In Press, J. Phys. A: Math. Theor. (2013
Mutually unbiased bases, orthogonal Latin squares, and hidden-variable models
Mutually unbiased bases encapsulate the concept of complementarity - the
impossibility of simultaneous knowledge of certain observables - in the
formalism of quantum theory. Although this concept is at the heart of quantum
mechanics, the number of these bases is unknown except for systems of dimension
being a power of a prime. We develop the relation between this physical problem
and the mathematical problem of finding the number of mutually orthogonal Latin
squares. We derive in a simple way all known results about the unbiased bases,
find their lower number, and disprove the existence of certain forms of the
bases in dimensions different than power of a prime. Using the Latin squares,
we construct hidden-variable models which efficiently simulate results of
complementary quantum measurements.Comment: Published versio
Dynamics of Atom-Atom Correlations in the Fermi problem
We present a detailed perturbative study of the dynamics of several types of
atom-atom correlations in the famous Fermi problem. This is an archetypal model
to study micro-causality in the quantum domain where two atoms, the first
initially excited and the second prepared in its ground state, interact with
the vacuum electromagnetic field. The excitation can be transferred to the
second atom via a flying photon and various kinds of quantum correlations
between the two are generated during this process. Among these, prominent
examples are given by entanglement, quantum discord and nonlocal correlations.
It is the aim of this paper to analyze the role of the light cone in the
emergence of such correlations.Comment: 14 pages, 7 figure
Affine Constellations Without Mutually Unbiased Counterparts
It has been conjectured that a complete set of mutually unbiased bases in a
space of dimension d exists if and only if there is an affine plane of order d.
We introduce affine constellations and compare their existence properties with
those of mutually unbiased constellations, mostly in dimension six. The
observed discrepancies make a deeper relation between the two existence
problems unlikely.Comment: 8 page
Foundations and Measures of Quantum Non-Markovianity
The basic features of the dynamics of open quantum systems, such as the
dissipation of energy, the decay of coherences, the relaxation to an
equilibrium or non-equilibrium stationary state, and the transport of
excitations in complex structures are of central importance in many
applications of quantum mechanics. The theoretical description, analysis and
control of non-Markovian quantum processes play an important role in this
context. While in a Markovian process an open system irretrievably loses
information to its surroundings, non-Markovian processes feature a flow of
information from the environment back to the open system, which implies the
presence of memory effects and represents the key property of non-Markovian
quantum behavior. Here, we review recent ideas developing a general
mathematical definition for non-Markoviantiy in the quantum regime and a
measure for the degree of memory effects in the dynamics of open systems which
are based on the exchange of information between system and environment. We
further study the dynamical effects induced by the presence of
system-environment correlations in the total initial state and design suitable
methods to detect such correlations through local measurements on the open
system.Comment: 31 pages, to be published in the special issue: Loss of coherence and
memory effects in quantum dynamics, Journal of Physics B: Atomic, Molecular
and Optical Physic
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