Compendium of Quantum Physics pp 205-209
Entropy of Entanglement
An essential feature of an entangled joint state (► entanglement) of two physical systems A, B is that the state of each subsystem is always mixed even though the joint state of the bipartite system may be pure. The entropy of the subsystems can therefore be used to quantify the entanglement of pure bipartite quantum states. For simplicity, we restrict ourselves to finite dimensions.
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