Discrimination of two-qubit unitaries via local operations and classical communication

TL;DR

This paper investigates the ability to distinguish between two-qubit bipartite unitaries using local operations and classical communication (LOCC), revealing limitations compared to global operations, especially in finite repetitions.

Contribution

It establishes conditions under which bipartite unitaries can be perfectly distinguished by LOCC, highlighting differences from global operations and asymptotic scenarios.

Findings

Entangling unitaries distinguishable by global operations are also distinguishable by LOCC.

Non-entangling unitaries can be distinguished globally but not by LOCC.

LOCC capabilities are more restricted than global operations in finite discrimination tasks.

Abstract

Distinguishability is a fundamental and operational task generally connected to information applications. In quantum information theory, from the postulates of quantum mechanics it often has an intrinsic limitation, which then dictates and also characterises capabilities of related information tasks. In this work, we consider discrimination between bipartite two-qubit unitary transformations by local operations and classical communication (LOCC) and its relations to entangling capabilities of given unitaries. We show that a pair of entangling unitaries which do not contain local parts, if they are perfectly distinguishable by global operations, can also be perfectly distinguishable by LOCC. There also exist non-entangling unitaries, e.g. local unitaries, that are perfectly discriminated by global operations but not by LOCC. The results show that capabilities of LOCC are strictly restricted than global operations in distinguishing bipartite unitaries for a finite number of repetitions, contrast to discrimination of a pair of bipartite states and also to asymptotic discrimination of unitaries.