Quantum discord D(A|B) = I(A:B) − J(A:B) measures quantum correlations beyond entanglement. Even separable states can have nonzero discord. Here we explore a two-qubit X-state ρ parameterized by mixing and coherence.
ρ = p·I/4 + (1-p)·|Ψ⟩⟨Ψ|
I(A:B) = S(A) + S(B) − S(AB)
D = I(A:B) − max_Π J(A:B|Π)
E_F = entanglement of formation
D ≥ 0, E_F = 0 possible