# Entropy of entanglement

The entropy of entanglement is an entanglement measure for a bipartite pure states. It is defined as the von Neumann entropy of one of the reduced states. That is, for a pure state ρAB = ∣Ψ⟩⟨Ψ∣AB, it is given by:

E(ρ) ≡ S(ρA) = S(ρB)
,

where ρA = TrB(∣Ψ⟩⟨Ψ∣) and ρB = TrA(∣Ψ⟩⟨Ψ∣).

Many entanglement measures reduce to the entropy of entanglement when evaluated on pure states. Among those are

Some entanglement measures that do not reduce to the entropy of entanglement are