Quantum discord measures quantum correlations beyond entanglement. A state can have zero entanglement yet nonzero discord — it contains "quantumness" that cannot be captured classically. Explore the correlation landscape for two-qubit state families.
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Concurrence (E)
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Quantum Discord (D)
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Mutual Info (I)
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Separable + no discord
Separable + discord
Entangled
Current state
Werner states:ρ = p|Φ⁺⟩⟨Φ⁺| + (1-p)I/4
Entangled for p > 1/3; discordant for p > 0.
Quantum discord:D(A:B) = I(A:B) − C(A:B)
where C(A:B) = max_{Π_k} [S(ρ_A) − Σ_k p_k S(ρ_A|k)] is the classical correlation under optimal measurement on B.
Discord captures quantum correlations in states that are not entangled — important for quantum computing without entanglement (DQC1 protocol).