Quantum Discord — Quantum Correlations Beyond Entanglement

Discord = total quantum correlations − classical correlations

Two-Qubit State ρ

Correlation Measures

Concurrence C = 0.00
Entanglement E_F = 0.00
Mutual Info I(A:B) = 0.00
Classical corr J = 0.00
Discord D = I−J = 0.00
Physics: Quantum discord (Ollivier & Zurek 2001, Henderson & Vedral 2001) measures quantum correlations beyond entanglement. For a bipartite state ρ_AB, the quantum mutual information I(A:B) = S(ρ_A)+S(ρ_B)−S(ρ_AB) = total correlations. The classical correlations J(A|B) = max over measurements on B of the accessible information. Discord D = I − J ≥ 0. A state can have D > 0 even when it is separable (unentangled). Discord vanishes iff ρ_AB is a "classical-quantum" state. Discord is connected to quantum computational advantage (DQC1 algorithm) and thermodynamic work extraction.