Quantum discord

The notion of quantum discord was introduced by Harold Ollivier and Wojciech H. Zurek[1][2] and, independently by Leah Henderson and Vlatko Vedral.

More specifically, quantum discord is the difference between two expressions which each, in the classical limit, represent the mutual information.

[10] Other conditions have been identified which can be seen in analogy to the Peres–Horodecki criterion[11] and in relation to the strong subadditivity of the von Neumann entropy.

[19] Discord can also be viewed in operational terms as an "entanglement consumption in an extended quantum state merging protocol".

[21][22] Non-linear classicality witnesses have been implemented with Bell-state measurements in photonic systems.

[29] At least for certain models of a qubit pair which is in thermal equilibrium and form an open quantum system in contact with a heat bath, the quantum discord increases with temperature in certain temperature ranges, thus displaying a behaviour that is quite in contrast with that of entanglement, and that furthermore, surprisingly, the classical correlation actually decreases as the quantum discord increases.

[32][33][34][35] An operational measure, in terms of distillation of local pure states, is the 'quantum deficit'.

[36] The one-way and zero-way versions were shown to be equal to the relative entropy of quantumness.

Faithful, computable and operational measures of discord-type correlations are the local quantum uncertainty[26] and the interferometric power.