10 research outputs found
Global Quantum Discord in Multipartite Systems
We propose a global measure for quantum correlations in multipartite systems,
which is obtained by suitably recasting the quantum discord in terms of
relative entropy and local von Neumann measurements. The measure is symmetric
with respect to subsystem exchange and is shown to be non-negative for an
arbitrary state. As an illustration, we consider tripartite correlations in the
Werner-GHZ state and multipartite correlations at quantum criticality. In
particular, in contrast with the pairwise quantum discord, we show that the
global quantum discord is able to characterize the infinite-order quantum phase
transition in the Ashkin-Teller spin chain.Comment: v3: 7 pages, 6 figures. Published versio
Monogamy of Quantum Discord by Multipartite Correlations
We introduce a monogamy inequality for quantum correlations, which implies
that the sum of pairwise quantum correlations is upper limited by the amount of
multipartite quantum correlations as measured by the global quantum discord.
This monogamy bound holds either for pure or mixed quantum states provided that
bipartite quantum discord does not increase under discard of subsystems. We
illustrate the monogamy behavior for multipartite pure states with Schmidt
decomposition as well as for W-GHZ mixed states. As a by-product, we apply the
monogamy bound to investigate residual multipartite correlations.Comment: 5 pages, 2 figures. v2: published versio
The sudden change phenomenon of quantum discord
Even if the parameters determining a system's state are varied smoothly, the
behavior of quantum correlations alike to quantum discord, and of its classical
counterparts, can be very peculiar, with the appearance of non-analyticities in
its rate of change. Here we review this sudden change phenomenon (SCP)
discussing some important points related to it: Its uncovering,
interpretations, and experimental verifications, its use in the context of the
emergence of the pointer basis in a quantum measurement process, its appearance
and universality under Markovian and non-Markovian dynamics, its theoretical
and experimental investigation in some other physical scenarios, and the
related phenomenon of double sudden change of trace distance discord. Several
open questions are identified, and we envisage that in answering them we will
gain significant further insight about the relation between the SCP and the
symmetry-geometric aspects of the quantum state space.Comment: Lectures on General Quantum Correlations and their Applications, F.
F. Fanchini, D. O. Soares Pinto, and G. Adesso (Eds.), Springer (2017), pp
309-33