Quantum Discord: A Measure of the Quantumness of Correlations

TLDR

Classical expressions for mutual information differ in quantum systems, revealing a fundamental distinction. The authors propose quantum discord as a metric for the quantumness of correlations, linking it to superposition and using its vanishing to identify pointer states. They relate discord to the quantum superposition principle and employ its nullity as a criterion for selecting effectively classical pointer states. Quantum discord is defined by the mutual information difference, and its persistence even in separable states demonstrates that lack of entanglement does not equate to classicality.

Abstract

Two classically identical expressions for the mutual information generally differ when the systems involved are quantum. This difference defines the quantum discord. It can be used as a measure of the quantumness of correlations. Separability of the density matrix describing a pair of systems does not guarantee vanishing of the discord, thus showing that absence of entanglement does not imply classicality. We relate this to the quantum superposition principle, and consider the vanishing of discord as a criterion for the preferred effectively classical states of a system, i.e., the pointer states.