Unambiguous discrimination of mixed states

We present the conditions under which probabilistic error-free discrimination of mixed states is possible, and provide upper and lower bounds on the maximum probability of success for the case of two mixed states. We solve certain special cases exactly, and demonstrate how the problems of state filtering and state comparison can be recast as problems of mixed state unambiguous discrimination.
Comment: 4.1 pages, crucial typos fixed. Final version, to be published in Phys. Rev. A (Rapid Comm)