Quantum Discord — Geometric Measure
Beyond entanglement: quantum discord captures all quantum correlations in a two-qubit state. Explore the landscape of classical vs quantum correlations.
Quantum Discord (Ollivier-Zurek 2001, Henderson-Vedral 2001): D = I(A:B) − C(A:B), where I is quantum mutual information and C is the maximum classical mutual information over all local measurements on B. For separable states, D can still be nonzero — discord captures quantum correlations beyond entanglement. Geometric discord = min distance to zero-discord states. Werner states: ρ = r|Ψ⁻⟩⟨Ψ⁻| + (1−r)I/4; discord > 0 for r > 0, entanglement only for r > 1/3.