Optimum Unambiguous Discrimination Between Linearly Independent   Symmetric States
  The quantum formalism permits one to discriminate sometimes between any set of linearly-independent pure states with certainty. We obtain the maximum probability with which a set of equally-likely, symmetric, linearly-independent states can be discriminated. The form of this bound is examined for symmetric coherent states of a harmonic oscillator or field mode. 
Anthony Chefles and Stephen M. Barnett