In quantum information theory, quantum discord is a measure of nonclassical correlations between two subsystems of a quantum system. It includes correlations that are due to quantum physical effects but do not necessarily involve quantum entanglement.

… quantum correlations can be present in certain mixed separable states; In other words, separability alone does not imply the absence of quantum effects. The notion of quantum discord thus goes beyond the distinction which had been made earlier between entangled versus separable (non-entangled) quantum states.

Nonzero quantum discord indicates the presence of correlations that are due to noncommutativity of quantum operators. For pure states, the quantum discord becomes a measure of quantum entanglement, more specifically, in that case it equals the entropy of entanglement.

Evidence has been provided for poignant differences between the properties of quantum entanglement and quantum discord. It has been shown that quantum discord is more resilient to dissipative environments than is quantum entanglement.

… surprisingly, the classical correlation actually decreases as the quantum discord increases.

— Wikipedia on Quantum discord

[Quantum discord is] the amount of entanglement needed in the task of state-merging.

— Jun 10 ’11 at 13:32

— Frederic Grosshans

2012.02.10 Friday ACHK