Finding decompositions of a class of separable states
Abstract
By definition a separable state has the form Σ Ai Bi, where 0 ≤ Ai, Bi for each i. In this paper we consider the class of states which admit such a decomposition with B1, ..., Bp having independent images. We give a simple intrinsic characterization of this class of states, and starting with a density matrix in this class, describe a procedure to find such a decomposition with B1, ..., Bp having independent images, and A1, ..., Ap being distinct with unit trace. Such a decomposition is unique, and we relate this to the facial structure of the set of separable states. A special subclass of such separable states are those for which the rank of the matrix matches one marginal rank. Such states have arisen in previous studies of separability (e.g., they are known to be a class for which the PPT condition is equivalent to separability). The states investigated also include a class that corresponds (under the Choi-Jamiokowski isomorphism) to the quantum channels called quantum-classical and classical-quantum by Holevo.