Entanglement, mixedness, and perfect local discrimination of orthogonal quantum states

It is shown that local distinguishability of orthogonal mixed states can be completely characterized by the local distinguishability of their supports irrespective of entanglement and mixedness of the states. This leads to two kinds of upper bounds on the number of locally distinguishable orthogonal mixed states. The first one depends only on pure-state entanglement within the supports of the states and therefore may be easy to compute in many instances. The second bound is optimal in the sense that it optimizes the bounding quantities, not necessarily the function of entanglement alone, over all orthogonal mixed-state ensembles (satisfying certain conditions) admissible within the supports of the density matrices.