Separable State - Separability For Mixed States

Separability For Mixed States

Consider the mixed state case. A mixed state of the composite system is described by a density matrix acting on . ρ is separable if there exist, and which are mixed states of the respective subsystems such that

where

Otherwise is called an entangled state. We can assume without loss of generality in the above expression that and are all rank-1 projections, that is, they represent pure ensembles of the appropriate subsystems. It is clear from the definition that the family of separable states is a convex set.

Notice that, again from the definition of the tensor product, any density matrix, indeed any matrix acting on the composite state space, can be trivially written in the desired form, if we drop the requirement that and are themselves states and If these requirements are satisfied, then we can interpret the total state as a probability distribution over uncorrelated product states.

In terms of quantum channels, a separable state can be created from any other state using local actions and classical communication while an entangled state cannot.

When the state spaces are infinite dimensional, density matrices are replaced by positive trace class operators with trace 1, and a state is separable if it can be approximated, in trace norm, by states of the above form.

If there is only a single non-zero, then the state is called simply separable (or it is called a "product state").