Quantum Discord & Bipartite Correlations

Beyond entanglement: quantum correlations in mixed states

Quantum Discord

Discord D(A|B) measures quantum correlations beyond classical ones. Separable states can have nonzero discord — entanglement is NOT the only quantum resource.

I(A:B) = S(A) + S(B) - S(AB) J(A|B) = S(A) - min_Π S(A|Π_B) Discord D(A|B) = I(A:B) - J(A|B) Werner state: ρ = p|Φ⁺⟩⟨Φ⁺| + (1-p)I/4

The Werner state is entangled for p > 1/3, but has nonzero discord for all p > 0 — quantum correlations survive even without entanglement.

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Mutual Info I
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Classical J
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Discord D
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Entanglement E