Complementarity Test with Unmeasured, Permanently Inaccessible Path Markers

Abstract

In prior complementarity tests, path markers were measured and remained causally accessible, leaving the collapse-locality loophole open. We address this loophole using a quantum eraser that isolates the encoding of path information in the joint quantum state from its later measurement and causal accessibility. We launch idler photons -- the sole path markers for entangled signal photons in an interferometer -- on outgoing null trajectories while the joint state remains in a coherent superposition of both interferometer path alternatives. Our flat $Λ$CDM transmission model predicts that 73-82% of launched idlers will propagate forever without absorption, scattering, or environmental path-record formation. For these modeled survivors, each path marker remains perpetually unmeasured, and propagation along its outgoing trajectory precludes later causal contact with its interferometric record. Unconditioned signal detections show no statistically significant launch-induced change in interference, as standard quantum theory predicts. Normalized to the modeled survivors, the 95% confidence upper bound on any launch-induced fringe is 0.16 of full restoration.