Optimal strategy to certify quantum nonlocality

TL;DR

This paper introduces an efficient method to identify optimal Bell inequalities for certifying quantum nonlocality, especially in challenging experimental conditions, improving detection and reducing loopholes.

Contribution

The authors develop a technique to find Bell inequalities with maximal quantum-classical gap tailored to specific measurement data, enhancing nonlocality certification in practical scenarios.

Findings

Improved detection of quantum nonlocality in weakly entangled photons.

Reduced photodetector efficiency needed to close detection loophole.

Validated method with experimental data showing enhanced certification.

Abstract

Certification of quantum nonlocality plays a central role in practical applications like device-independent quantum cryptography and random number generation protocols. These applications entail the challenging problem of certifying quantum nonlocality, something that is hard to achieve when the target quantum state is weakly entangled, or when the source of errors is high, e.g. when photons propagate through the atmosphere or a long optical fiber. Here, we introduce a technique to find a Bell inequality with the largest possible gap between the quantum prediction and the classical local hidden variable limit for a given set of measurement frequencies. Our method represents an efficient strategy to certify quantum nonlocal correlations from experimental data without requiring extra measurements, in the sense that there is no Bell inequality with a larger gap than the one provided. Furthermore, we also reduce the photodetector efficiency required to close the detection loophole. We illustrate our technique by improving the detection of quantum nonlocality from experimental data obtained with weakly entangled photons.