Quantum Discord

Beyond entanglement: quantum correlations in mixed states

Mixing p: 0.50 Entanglement θ: 45°

Entropy S(ρ)

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Mutual Info I

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Classical Corr J

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Discord D

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Entanglement E

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Quantum discord D = I(A:B) − J(A:B) measures quantum correlations beyond entanglement. Even separable mixed states can have nonzero discord. Introduced by Ollivier & Zurek (2001), discord captures the "quantumness" of correlations accessible only by disturbing the system — relevant to quantum advantage in computation without entanglement.