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arXiv:1012.0632 · Journal of Physics A Mathematical and Theoretical

Generalizations of quantum discord

Jianwei Xu

Published Oct 18, 2011• 6 citations
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Abstract

The original definition of quantum discord of bipartite states was defined over projective measurements; in this paper we discuss some generalizations of it. These generalizations are defined over general measurements, rank-1 general measurements or Neumark extension measurements. We investigate the non-negativity, zero-discord sets of all these quantum discords and some properties of them.

DOI
10.1088/1751-8113/44/44/445310

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The original definition of quantum discord of bipartite states was defined over projective measurements; in this paper we discuss some generalizations of it. These generalizations are defined over general measurements, rank-1 general measurements or Neumark extension measurements. We investigate the non-negativity, zero-discord sets of all these quantum discords and some properties of them.
The original definition of quantum discord of bipartite states was defined under one-side projective measurements, it describes quantum correlation more extensively than entanglement. Dakic, Vedral, and Brukner [Phys. Rev. Lett. \textbf{105}, 190502 (2010)] introduced a geometric measure of quantum discord, and Luo, Fu [Phys. Rev. A \textbf{82}, 034302 (2010)] simplified the expression of it. In this paper we generalize the quantum discord to the case of two-side projective measurements, and also define a geometric measure on it. Further, a simplified expression and a lower bound of this geometric measure are derived and explicit expressions are obtained for some special cases.
A generalization of quantum discord to multipartite systems is proposed. A key feature of our formulation is its consistency with the conventional definition of discord in bipartite systems. It is by construction zero only for systems with classically correlated subsystems and is a non-negative quantity, giving a measure of the total nonclassical correlations in the multipartite system with respect to a fixed measurement ordering. For the tripartite case, we show that the discord can be decomposed into contributions resulting from changes induced by nonclassical correlation breaking measurements in the conditional mutual information and tripartite mutual information. The former gives a measure of the bipartite nonclassical correlations and is a non-negative quantity, while the latter is related to the monogamy of the nonclassical correlations.
Due to some ambiguity in the definition of mutual Tsallis entropy in classical probability theory, its generalization to quantum theory is discussed and, as a consequence, two types of generalized quantum discords, called q-discords, are defined in terms of quantum Tsallis entropy. Both q-discords for two-qubit Werner and isotropic states are determined and compared and it is shown that one of them is non-negative, at least for states under investigation, for all q > 0. Finally, an analytical expression for q-discord of certain family of two-qubit X-states is presented. Using this example, we show that both types of q-discords can take negative values for some q > 1, hence their use as correlations measures is rather limited.
We extend the definition of quantum discord as a quantifier of nonClassical correlations in a quantum state to the case where weak measurements are performed on subsystem AAA of a bipartite system ABABAB. The properties of weak discord are explored for several families of quantum states. We find that in many cases weak quantum discord is identical to normal discord and in general the values of the two are very close to each other. Weak quantum discord reduces to discord in the appropriate limits as well. We also discuss the implications of these observations on the interpretations of quantum discord.
We investigate quantum states that possess both maximum entanglement and maximum discord between the pertinent parties. Since entanglement (discord) is defined only for bipartite (two-qubit) systems, we use an appropriate sum over all bipartitions as the associated measure. The ensuing definition---not new for entanglement---is thus extended here to quantum discord. Also, additional dimensions within the parties are considered (qudits). We also discuss quantum correlations that induce Mermin's Bell-inequality violation for all multiqubit systems. One finds some differences when quantum mechanics is defined over the field of real or of complex numbers.