Loss-induced anomalous generalized bunching in multiphoton interference

Abstract

We show that internal loss and survival conditioning can activate anomalous generalized bunching in passive linear optical circuits. We introduce a conditional bunching probability that all photons occupy a target region of accessible output modes, given that all photons survive. For two-photon inputs, we prove that this probability is always monotonic for any circuit size and loss configuration, although a multimode target region can reverse the monotonic direction. For three-photon inputs in a minimal three-mode lossy interferometer, we find a nonmonotonic anomaly in which the conditional bunching probability is maximized for partially distinguishable photons. This behavior is forbidden for the corresponding unconditioned target-region probability, demonstrating that survival-conditioned loss changes the minimal hierarchy of generalized bunching.