Karol Bartkiewicz, Karel Lemr, Antonín Černoch, Jan Soubusta
The threshold between classical and nonclassical two-qubit states is drawn at the place when those states can no longer be described by classical correlations, i.e., quantum discord or entanglement appear. However, to check if the correlations are classical (in terms of quantum discord and entanglement) it is sufficient to witness the lack of quantum discord because its zero value implies the lack of entanglement. In this Rapid Communication we introduce a quantum discord witness which is able to detect quantum discord for a wide variety of states including symmetric states and is a universal classicality witness up to local unitary transformations. We also discuss how this witness can be practically measured in linear-optical systems using standard beam splitters and photon detectors. We study the efficiency of the setup assuming both ideal and real components and show that the efficiency of the proposed implementation is better than the full two-photon quantum tomography. We expect that a similar approach to the presented one can be used to directly measure quantum discord. The variance-like mathematical form of the discovered witness can be a manifestation of a nontrivial physical phenomenon possibly related to quantum phase transitions.
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http://arxiv.org/abs/1302.1221
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