Bell-GHZ nonclassicality of many-observer interwoven frustrated down conversions

Frustrated down conversion is a process in which a quantum superposition of emissions from two separate parametric down-conversion processes gives rise to observable interference. Depending on the phase relation between the probability amplitudes associated with emissions by the first and second crystal, the process can be enhanced or suppressed. This is achieved by aligning the setup so that the signal and idler modes from the first crystal are fed into the second and constitute its signal-idler modes. In Sci. Adv. 11, 1794 (2025), two-observer interwoven frustrated PDC processes produced interference effects based on path identity [Phys. Rev. Lett. 118, 080401 (2017)]. The signal and idler modes of source crystals I and II are arranged to fully overlap with the emission modes of crystals A and B, which serve as elements of measurement stations controlled by Alice and Bob. In the interwoven configuration, crystal A (B) receives the signal mode of crystal I (II) and the idler mode of crystal II (I), enabling interference between joint emission processes at the sources and at the measurement stations. It was conjectured that such interference may lead to new non-classical phenomena. In arXiv:2508.19207 it was shown that the process violates the standard Clauser-Horne Bell inequality without additional assumptions, provided suitable measurement settings are used. Here we extend the interference scheme to more than two measurement stations and demonstrate a violation of one of the WWWZB inequalities. This indicates that the proposed approach may provide a general method for revealing non-classicality in a range of phenomena discussed in [Rev. Mod. Phys. 94, 025007 (2022)]. We also present a GHZ/Hardy-type argument that further highlights the paradoxical character of the interference.