Classification of joint quantum measurements based on entanglement cost of localization

TL;DR

This paper classifies two-qubit joint quantum measurements based on the finite entanglement needed for their localization, revealing new insights into measurement complexity and entanglement resources.

Contribution

It introduces a systematic classification of joint measurements by entanglement cost and develops finite-resource teleportation schemes for their localization.

Findings

Classified all two-qubit measurements localizable with finite entanglement.

Identified measurements with special properties, like Bell state measurement.

Proposed methods for exploring higher-dimensional and multipartite measurements.

Abstract

Despite their importance in quantum theory, joint quantum measurements remain poorly understood. An intriguing conceptual and practical question is whether joint quantum measurements on separated systems can be performed without bringing them together. Remarkably, by using shared entanglement, this can be achieved perfectly when disregarding the post-measurement state. However, existing localization protocols typically require unbounded entanglement. In this work, we address the fundamental question: "Which joint measurements can be localized with a finite amount of entanglement?" We develop finite-resource versions of teleportation-based schemes and analytically classify all two-qubit measurements that can be localized in the first steps of these hierarchies. These include several measurements with exceptional properties and symmetries, such as the Bell state measurement and the elegant joint measurement. This leads us to propose a systematic classification of joint measurements based on entanglement cost, which we argue directly connects to the complexity of implementing those measurements. We illustrate how to numerically explore higher levels and construct generalizations to higher dimensions and multipartite settings.