  The problem of quantum state discrimination between two wave functions on a
ring is considered. The optimal minimum-error probability is known to be given
by the Helstrom bound. A new strategy is introduced by inserting either
adiabatically or instantaneously an impenetrable barrier. The insertion point,
independent of the shape of the initial wave function, becomes a node. The
resulting modified wave functions can be, if the initial functions are
judiciously chosen, distinguished with a smaller error probability, and as a
consequence the Helstrom bound can be violated under idealised conditions.
