Generalized measurement incompatibility

TL;DR

This paper introduces a generalized notion of measurement incompatibility called partial joint-measurability, providing mathematical formulations, decision procedures, and implications for quantum cryptography security.

Contribution

It extends the concept of joint-measurability to subsets of outcomes, offers a semidefinite program for decision, and analyzes security thresholds in quantum cryptography.

Findings

Partial joint-measurability can be decided via a single semidefinite program.

An adversary limited to classical information can guess outcomes if measurements are partially jointly measurable.

Detection efficiency thresholds for partial joint-measurability are derived, impacting quantum cryptography security.

Abstract

Quantum measurements can be incompatible, i.e., they can fail to be jointly measurable. Recently, a weaker notion of joint-measurability, called partial joint-measurability, was proposed by Masini et al. in [Quantum 8, 1574 (2024)]. In this work, we further generalize this notion to the setting where only a subset of the outcomes of each measurement is required to be jointly determined by classical variables. We provide two mathematical formulations of partial joint-measurability and show that, like full joint-measurability, it can be decided by solving a single semidefinite program. We prove that in the case of an untrusted measurement device, an adversary Eve, limited to classical side information, can perfectly guess the outcomes of the measurement device if and only if the set of measurements is partially jointly measurable. We derive analytical thresholds on the detection efficiency below which generic measurements become partially jointly measurable. Such bounds directly yield limits on the robustness of device-independent and semi-device-independent quantum cryptographic protocols against detection inefficiency. In particular, our results highlight the importance of a careful treatment of postselection in security analyses.