Quantum discord D(A|B) = I(A:B) − C(A:B) measures quantum correlations that survive even in separable (unentangled) mixed states. Unlike entanglement, discord captures the non-commutativity of local measurements with the state. A Werner state ρ = p|Ψ⁻⟩⟨Ψ⁻| + (1-p)I/4 is entangled for p>1/3 but has positive discord for all p>0.
ρ(p) = p|Ψ⁻⟩⟨Ψ⁻| + (1-p)I/4
D = I(A:B) − max_measurements[H(A) − H(A|B)]