Unquestionable Bell Theorem for Interwoven Frustrated Downconversion Processes

Interwoven frustrated parametric down-conversion (PDC) processes produce interference effects based on path identity [Krenn et al., Entanglement by path identity, Phys. Rev. Lett. 118, 080401 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.080401]. 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 nonstandard approach is needed because if only the local phase shifts are used for the measurement settings, as done in the experiment reported in Wang et al. [Violation of Bell inequality with unentangled photons, Sci. Adv. 11, eadr1794 (2025)SACDAF2375-254810.1126/sciadv.adr1794], 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 nonclassicality 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.