Non-classical correlations beyond entanglement
Quantum discord D(A|B) = I(A:B) − J(A:B) captures quantum correlations not captured by classical mutual information. Even separable (unentangled) states can have non-zero discord. Projective measurement on subsystem B collapses the joint state, demonstrating measurement backaction. The Bloch sphere trajectories show how the state evolves under coupling and decoherence.