  We address the problem of unambiguously identifying the state of a probe
qudit with the state of one of d reference qudits. The reference states are
assumed pure and linearly independent but we have no knowledge of them. The
state of the probe qudit is assumed to coincide equally likely with either one
of the d unknown reference states. We derive the optimum measurement strategy
that maximizes the success probability of unambiguous identification and find
that the optimum strategy is a generalized measurement. We give both the
measurement operators and the optimum success probability explicitly.
Technically, the problem we solve amounts to the optimum unambiguous
discrimination of d known mixed quantum states.
