The entanglement entropy can be expressed using the singular values of the Schmidt decomposition of the state. Any pure state can be written as | Ψ Ψ ⟩ ⟩ = ∑ ∑ i = 1 m α α i | u i ⟩ ⟩ A ⊗ ⊗ | v i ⟩ ⟩ B where | u i ⟩ ⟩ A and | v i ⟩ ⟩ B are orthonormal states in subsystem A and subsystem B respectively. The entropy of entanglement is simply: