QUANTUMNESS OF CORRELATIONS AND ENTANGLEMENT
A. R. Usha Devi, A. K. Rajagopal, SUDHA
Generalized measurement schemes on one part of bipartite states, which would leave the set of all separable states insensitive are explored here to understand quantumness of correlations in a more general perspective. This is done by employing linear maps associated with generalized projective measurements. A generalized measurement corresponds to a quantum operation mapping a density matrix to another density matrix, preserving its positivity, hermiticity, and trace class. The positive operator valued measure (POVM) — employed earlier in the literature to optimize the measures of classical/quantum correlations — correspond to completely positive (CP) maps. The other class, the not completely positive (NCP) maps, are investigated here, in the context of measurements, for the first time. It is shown that such NCP projective maps provide a new clue to the understanding of quantumness of correlations in a general setting. Especially, the separability–classicality dichotomy gets resolved only when both the classes of projective maps (CP and NCP) are incorporated as optimizing measurements. An explicit example of a separable state — exhibiting nonzero quantum discord, when possible optimizing measurements are restricted to POVMs — is reexamined with this extended scheme incorporating NCP projective maps to elucidate the power of this approach.
arXiv:1105.4115•International Journal of Quantum Information•Oct 1, 2011•10 citations