Separating quantum from classical correlations in bipartite mixed states
Quantum discord (Ollivier & Zurek 2001, Henderson & Vedral 2001): D(A|B) = I(A:B) − J(A:B), where I(A:B) = S(A)+S(B)−S(AB) is mutual information and J(A:B) = max_{measurements on B} [S(A)−S(A|B_meas)] is classical correlations. Discord = quantum correlations beyond entanglement. Separable (non-entangled) states can have nonzero discord! Only classically correlated states (pointer states) have D=0.