Quantum Discord — Beyond Entanglement

Ollivier-Zurek 2001: quantum correlations that survive separable states
Mutual Info I(A:B)
0.000
Classical Corr J(A:B)
0.000
Discord D(A|B)
0.000
Entanglement E
0.000
0.50
0.00
0.25
Quantum discord D(A|B) = I(A:B) − J(A:B) measures quantum correlations beyond entanglement. I(A:B) is total mutual information; J(A:B) is the maximum classical information extractable by local measurements on B. Discord can be nonzero for separable (non-entangled) states, and can exist even when entanglement is fully destroyed by dephasing. The Werner family and two-qubit X-states are explored here. The bar chart shows how discord, classical correlations, and entanglement trade off as parameters change. Discord ≈ 0 only for classically correlated (pointer) states.