Unquestionable Bell theorem for interwoven frustrated down conversion processes

TL;DR

This paper demonstrates Bell nonclassicality in interwoven frustrated parametric down conversion processes by violating the Clauser-Horne inequality using on-off switching measurements, establishing a new platform for quantum phenomena.

Contribution

It introduces a novel Bell test approach with on-off switching in PDC processes, proving non-classicality where phase-only measurements fail.

Findings

Violates Clauser-Horne inequality with on-off switching

Shows deep destructive interference can violate Bell inequalities

Establishes a new platform for exploring quantum phenomena

Abstract

Interwoven frustrated parametric down conversion (PDC) processes produce interference effects based on path identity [Phys. Rev. Lett. 118, 080401 (2017)]. In this letter we show Bell nonclassicality of the processes: a proper violation of the Clauser-Horne inequality when the local measurements are controlled by on-off switching of the final local PDC processes. Such a non-standard approach is needed because if only the local phase shifts are used for the measurement settings, as done in the experiment reported in Sci. Adv. 11, 1794 (2025), there exists a local realistic model of the interference, which we present in this letter. Nevertheless, the reported destructive interference is deep enough to violate the inequality when using on-off switching, so our result establishes a firm footing for non-classicality of the new interferometry, in both theory and experiment. The on-off approach to Bell analysis of path-identity-based interference forms a new platform for seeking new highly counterintuitive quantum phenomena.