  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 instantaneously two
impenetrable barriers dividing the ring into two chambers. In the process, the
candidate wave functions, as the insertion points become nodes, get entangled
with the barriers and can, if judiciously chosen, be distinguished with smaller
error probability. As a consequence, the Helstrom bound under idealised
conditions can be violated.
