According to the Hong-Ou-Mandel effect, two indistinguishable photons impinging on a balanced beam splitter are always detected together in the same output mode as a result of the destructive suppression of coincidence events. This boson-bunching effect is weakened as the photons become more distinguishable (e.g., via differences in polarization, arrival time, etc.). In linear interference experiments with multiple photons, this trend is commonly admitted to reflect a general rule, namely that bunching is maximized for indistinguishable photons and gradually declines as photons become distinguishable. In this talk, we show that this general rule is false by providing explicit examples in which certain bunching probabilities are maximized by the input states of partially distinguishable photons. Interestingly, such examples of anomalous bunching are tightly related to recently found counterexamples to long-standing conjectures on matrix permanents. We will also present a three-photon experiment in which pairwise indistinguishability is increased, yet the probability that all photons coalesce into the same output is decreased due to collective photonic phases. These results highlight the complex behaviour of multiparticle quantum interference in the grey zone between ideal bosons and classical particles.
