Quantum Discord (Ollivier & Zurek 2001; Henderson & Vedral 2001) measures nonclassical correlations in bipartite quantum states that go beyond entanglement. For a state ρAB:
D(A|B) = I(A:B) − J(A|B)
where I(A:B) = S(A) + S(B) − S(AB) is the quantum mutual information (S = von Neumann entropy) and J(A|B) is the maximum classical correlation accessible via measurements on B. A state has zero discord if and only if it is classical-quantum (one subsystem has a preferred basis). Separable states can have nonzero discord — discord is strictly more general than entanglement. States with nonzero discord can still provide quantum computational advantage (DQC1 protocol). The sliders explore the X-state family of two-qubit density matrices, where discord has closed-form expressions.