353 research outputs found
Robustness of Entanglement as a Resource
The robustness of multipartite entanglement of systems undergoing decoherence
is of central importance to the area of quantum information. Its
characterization depends however on the measure used to quantify entanglement
and on how one partitions the system. Here we show that the unambiguous
assessment of the robustness of multipartite entanglement is obtained by
considering the loss of functionality in terms of two communication tasks,
namely the splitting of information between many parties and the teleportation
of states.Comment: 11 pages, 5 figure
Unifying approach to the quantification of bipartite correlations by Bures distance
The notion of distance defined on the set of states of a composite quantum
system can be used to quantify total, quantum and classical correlations in a
unifying way. We provide new closed formulae for classical and total
correlations of two-qubit Bell-diagonal states by considering the Bures
distance. Complementing the known corresponding expressions for entanglement
and more general quantum correlations, we thus complete the quantitative
hierarchy of Bures correlations for Bell-diagonal states. We then explicitly
calculate Bures correlations for two relevant families of states: Werner states
and rank-2 Bell-diagonal states, highlighting the subadditivity which holds for
total correlations with respect to the sum of classical and quantum ones when
using Bures distance. Finally, we analyse a dynamical model of two independent
qubits locally exposed to non-dissipative decoherence channels, where both
quantum and classical correlations measured by Bures distance exhibit freezing
phenomena, in analogy with other known quantifiers of correlations.Comment: 18 pages, 4 figures; published versio
Quantum optics in the phase space - A tutorial on Gaussian states
In this tutorial, we introduce the basic concepts and mathematical tools
needed for phase-space description of a very common class of states, whose
phase properties are described by Gaussian Wigner functions: the Gaussian
states. In particular, we address their manipulation, evolution and
characterization in view of their application to quantum information.Comment: Tutorial. 23 pages, 1 figure. Updated version accepted for
publication in EPJ - ST devoted to the memory of Federico Casagrand
Quantum Correlations, Chaos and Information
Quantum chaos is the study of quantum systems whose classical description is
chaotic. How does chaos manifest itself in the quantum world? In this spirit,
we study the dynamical generation of entanglement as a signature of chaos in a
system of periodically kicked coupled-tops, where chaos and entanglement arise
from the same physical mechanism. The long-time entanglement as a function of
the position of an initially localized wave packet very closely correlates with
the classical phase space surface of section - it is nearly uniform in the
chaotic sea, and reproduces the detailed structure of the regular islands. The
uniform value in the chaotic sea is explained by the random state conjecture.
As classically chaotic dynamics take localized distributions in phase space to
random distributions, quantized versions take localized coherent states to
pseudo-random states in Hilbert space. Such random states are highly entangled,
with an average value near that of the maximally entangled state. Continuing on
our journey to find the footprints of chaos in the quantum world, we explore
quantum signatures of classical chaos by studying the rate of information gain
in quantum tomography. We find an increase in the rate of information gain and
hence higher fidelities in the process when the Floquet maps employed increase
in chaoticity. We make predictions for the information gain using random matrix
theory in the fully chaotic regime and show a remarkable agreement between the
two. The last part of this thesis is devoted to the study of the nature of
quantum correlations themselves. We present an operational interpretation of
quantum discord based on the quantum state merging protocol. Quantum discord is
the markup in the cost of quantum communication in the process of quantum state
merging, if one discards relevant prior information.Comment: Dissertatio
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