Enhancing Quantum State Discrimination with Indefinite Causal Order

TL;DR

This paper explores how indefinite causal order, via quantum switch and superswitches, can improve quantum state discrimination in noisy and unknown channel conditions, surpassing traditional methods.

Contribution

It introduces the use of indefinite causal order protocols, including superswitches, to enhance quantum state discrimination under noise and uncertainty.

Findings

Guessing probability improves with indefinite causal order.

Superswitches outperform standard discrimination methods.

Certain channels benefit significantly from this approach.

Abstract

The standard quantum state discrimination problem can be understood as a communication scenario involving a sender and a receiver following these three steps: (i) the sender encodes information in pre-agreed quantum states, (ii) sends them over a noiseless channel, and (iii) the receiver decodes the information by performing appropriate measurements on the received states. In a practical setting, however, the channel is not only noisy but often also unknown, thus altering the states and making optimal decoding generally not possible. In this work, we study this noisy discrimination scenario using a protocol based on indefinite causal order. To this end, we consider the quantum switch and define its higher-order generalisations, which we call superswitches. We find that, for certain channels and ensembles, the guessing probability can be significantly improved compared to both single- and multi-copy state discrimination.