Separable quantum state

A bipartite state is separable when it is a convex combination ${\sum }_j{p}_j{\rho }_j^A\otimes {\rho }_j^B$ of product states; otherwise it is entangled.

Every separable state has positive partial transpose. The condition is also sufficient for separability on $2\otimes 2$ and $2\otimes 3$ systems.

A channel is entanglement breaking when applying it to one side of every bipartite state always produces a separable state. Equivalently, its Choi matrix is separable, and equivalently it has a measure-and-prepare representation.