In the absence of quantum entanglement, the quantum state of the composite system is called a separable state. The density matrix of a bipartite system in a separable state can be expressed as ρ ρ = ∑ ∑ k p k ρ ρ 1 k ⊗ ⊗ ρ ρ 2 k , where ∑ ∑ k p k = 1 . If there is only a single non-zero p k , then the state can be expressed just as ρ ρ = ρ ρ 1 ⊗ ⊗ ρ ρ 2 , and is called simply separable or product state.