Discord D = I(A:B) − J(A:B) measures quantum correlations in separable states. For ρ = p|Φ⁺⟩⟨Φ⁺| + (1−p)|00⟩⟨00|, discord persists even when entanglement vanishes.
State Parameters
Mutual Info I
—
Classical J
—
Discord D
—
Concurrence C
—
Separability boundary: p = 1/3
Discord > 0 for all p > 0
Entanglement C > 0 only for p > 1/3
Key insight: Quantum advantage in computation can come from discord alone — no entanglement needed.