The Helstrom Bound

TL;DR

This paper proposes a novel quantum state discrimination strategy on a ring that, under ideal conditions, can surpass the traditional Helstrom bound by entangling wave functions with barriers during their insertion.

Contribution

It introduces a new method involving barrier insertion and entanglement to reduce error probability in quantum state discrimination beyond the Helstrom limit.

Findings

The strategy can violate the Helstrom bound under ideal conditions.

Entanglement with barriers improves discrimination accuracy.

The method offers a new approach to quantum measurement limits.

Abstract

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.