Operational interpretations of quantum discord

Quantum discord quantifies nonclassical correlations beyond the standard classification of quantum states into entangled and unentangled. Although it has received considerable attention, it still lacks any precise interpretation in terms of some protocol in which quantum features are relevant. Here we give quantum discord its first information-theoretic operational meaning in terms of entanglement consumption in an extended quantum-state-merging protocol. We further relate the asymmetry of quantum discord with the performance imbalance in quantum state merging and dense coding.

Figure 1

Starting from a tripartite state ${\psi }_{\mathrm{ABC}}$, the goal of SM is to transfer Alice’s (A) part of the state to Bob (B), possibly using some extra entanglement or having some entanglement left over. The total entanglement consumption in this process is equal to $D(A|C)$ [see Eq. (4)].