Device-independent certification of non-classical joint measurements via causal models

TL;DR

This paper introduces a device-independent method to certify non-classical joint quantum measurements by analyzing measurement statistics against classical causal models, surpassing previous post-selection-based approaches.

Contribution

It proposes a new notion of measurement non-classicality based on incompatibility with classical causal models, enabling more precise certification.

Findings

Provides a causal model framework for measurement certification

Demonstrates non-classicality beyond post-selection methods

Enhances understanding of quantum measurement non-classicality

Abstract

Quantum measurements are crucial for quantum technologies and give rise to some of the most classically counter-intuitive quantum phenomena. As such, the ability to certify the presence of genuinely non-classical joint measurements in a device-independent fashion is vital. However, previous work has either been non-device-independent, or has relied on post-selection---the ability to discard all runs of an experiment in which a specific event did not occur. In the case of entanglement, the post-selection approach applies an entangled measurement to independent states and post-selects the outcome, inducing non-classical correlations between the states that can be device-independently certified using a Bell inequality. That is, it certifies measurement non-classicality not by what it is, but by what it does. This paper remedies this discrepancy by providing a novel notion of what measurement non-classicality is, which, in analogy with Bell's theorem, corresponds to measurement statistics being incompatible with an underlying classical causal model. It is shown that this provides a more fine-grained notion of non-classicality than post-selection, as it certifies the presence of non-classicality that cannot be revealed by examining post-selected outcomes alone.