  Quantum correlations do not allow signalling, and any operation which may be
performed on one system of an entangled pair cannot be detected by measurement
of the other system alone. This no-signalling condition limits allowed
operations and, in the context of quantum communication, may be used to put
bounds on quantum state discrimination. We find that the natural figure of
merit to consider is the confidence in identifying a state, which is optimised
by the maximum confidence strategy. We show that this strategy may be derived
from the no-signalling condition, and demonstrate the relationship between
maximum confidence measurements and entanglement concentration.
