Unquestionable Bell theorem for interwoven frustrated down conversion processes

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.