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Communications in Theoretical Physics

Quantum Correlations in a Family of Two-Qubit Separable States

Biao‐Liang Ye, Yi-Min Liu, Chunjie Xu, Xiansong Liu, Zhanjun Zhang

Published Sep 1, 2013• 8 citations
Publisher

Abstract

Quantum correlations in a family of two-qubit separable classical-quantum correlated states are intensively studied with four different approaches, namely, quantum discord [Phys. Rev. Lett. 88 (2002) 017901], measurement-induced disturbance (MID) [ Phys. Rev. A 77 (2008) 022301 ], ameliorated MID [ J. Phys. A: Math. Theor. 44 (2011) 352002 ] and quantum dissonance [ Phys. Rev. Lett. 104 (2010) 080501 ]. Quantum correlations captured with different approaches are compared and discussed so that their three distinct features are exposed.

DOI
10.1088/0253-6102/60/3/05

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Quantum correlations in a family of two-qubit separable classical-quantum correlated states are intensively studied with four different approaches, namely, quantum discord [Phys. Rev. Lett. 88 (2002) 017901], measurement-induced disturbance (MID) [ Phys. Rev. A 77 (2008) 022301 ], ameliorated MID [ J. Phys. A: Math. Theor. 44 (2011) 352002 ] and quantum dissonance [ Phys. Rev. Lett. 104 (2010) 080501 ]. Quantum correlations captured with different approaches are compared and discussed so that their three distinct features are exposed.
Quantum correlations in a family of bipartite separable qubit-qutrit quantum-classical correlated states are investigated by using two popular measures, i.e., the original quantum discord (OQD) method by Ollivier and Zurek [ Phys. Rev. Lett. 88 (2001) 017901 ] and the measurement-induced disturbance (MID) method by Luo [ Phys. Rev. A 77 (2008) 022301 ]. It is found that both of them are functions of a parameter partially characterizing the concerned states, however, quantum correlations evaluated via the convenient MID method are somewhat overestimated.
Quantum discord, as introduced by Olliver and Zurek [Phys. Rev. Lett. 88, 017901 (2001)], is a measure of the discrepancy between two natural yet different quantum analogs of the classical mutual information. This notion characterizes and quantifies quantumness of correlations in bipartite states from a measurement perspective, and is fundamentally different from the various entanglement measures in the entanglement vs separability paradigm. The phenomenon of nonzero quantum discord is a manifestation of quantum correlations due to noncommutativity rather than due to entanglement, and has interesting and significant applications in revealing the advantage of certain quantum tasks. We will evaluate analytically the quantum discord for a large family of two-qubit states, and make a comparative study of the relationships between classical and quantum correlations in terms of the quantum discord. We furthermore compare the quantum discord with the entanglement of formation, and illustrate that the latter may be larger than the former, although for separable states, the entanglement of formation always vanishes and thus is less than the quantum discord.
Quantum discord in any mixture of two bi-qubit arbitrary product states is studied with two different approaches. In the first approach the maximal classical correlations are obtained via numerical computations, while in the second approach they are analytically derived. Quantum correlations captured with both approaches completely coincide, as is in accord with the conclusion of Cen et al. [Phys. Rev. A 83 (2011) 054101]. The symmetry reduction of the concerned states concerning quantum correlations is accomplished. The captured discords are amply analyzed so that some inherent distinct properties are revealed.
Quantum discord, a kind of quantum correlation, is defined as the difference between quantum mutual information and classical correlation in a bipartite system. In general, this correlation is different from entanglement, and quantum discord may be nonzero even for certain separable states. Even in the simple case of bipartite quantum systems, this different kind of quantum correlation has interesting and significant applications in quantum information processing. So far, quantum discord has been calculated explicitly only for a rather limited set of two-qubit quantum states and expressions for more general quantum states are not known. In this article, we derive explicit expressions for quantum discord for a larger class of two-qubit states, namely, a seven-parameter family of so called XXX states that have been of interest in a variety of contexts in the field. We also study the relation between quantum discord, classical correlation, and entanglement for a number of two-qubit states to demonstrate that they are independent measures of correlation with no simple relative ordering between them.
Measure-induced non-classicality( MINC) in a family of bipartite separable qubit-qutrit correlated states was evaluated by using the MID method proposed by the reference [Luo S L. Using measurement-induced disturbance to characterize correlations as classical or quantum[J]. Phys Rev A,2008,77: 022301]. Specifically,classical correlation in each state was derived via the spectrum resolution technique and then the MINC( i. e.,quantum correlation) was extracted. The MINC was amply analyzed so that some distinct features were exposed.
Quantum discord provides a measure for quantifying quantum correlations beyond entanglement and is very hard to compute even for two-qubit states because of the minimization over all possible measurements. Recently a simple algorithm to evaluate the quantum discord for two-qubit XXX states was proposed by Ali, Rau, and Alber [Phys. Rev. A 81, 042105 (2010)] with minimization taken over only a few cases. Here we shall at first identify a class of XXX states, whose quantum discord can be evaluated analytically without any minimization, for which their algorithm is valid, and also identify a family of XXX states for which their algorithm fails. And then we demonstrate that this special family of XXX states provides furthermore an explicit example for the inequivalence between the minimization over positive operator-valued measures and that over von Neumann measurements.