Experimental Proposal: Testing Frame Width of Coupling in Entangled Photon Correlations

This proposal outlines a high-resolution Hong–Ou–Mandel (HOM) interferometry experiment designed to test whether the coincidence-versus-delay profile contains a compact-support contribution beyond standard joint spectral amplitude (JSA) physics. Using a femtosecond-pumped type-II SPDC source (ppKTP, 405/810 nm) and a stabilized free-space delay line scanned in 10-fs steps, we compare measured HOM dips against two model classes: (i) conventional JSA-based envelopes (Gaussian, sinc-derived, chirped, and measured-JSA models), and (ii) a phenomenological “Frame Width of Coupling” (FWC) class where the envelope is convolved with a finite rectangular kernel. The FWC model predicts a flat-bottomed dip with an approximately invariant plateau width under filter, pump-power, and chirp sweeps—distinct from standard coherence-time scaling. The proposal provides a falsifiable experimental design, quantitative discriminants (AIC/BIC, edge-steepness metrics, invariance tests), feasibility estimates, and simulation code. The experiment is intended for community replication and serves either to reveal compact-support structure or to place upper bounds on any such contribution at the 20–50 fs scale.