Observation-dependent suppression and enhancement of two-photon coincidences by tailored losses.

The ability of indistinguishable particles to interfere with one another is a core principle of quantum mechanics. The interplay of interference and particles exchange statistics1–4 gives rise to the Hong–Ou–Mandel (HOM) effect5, where the bunching of bosons suppresses two-particle coincidences between the output ports of a balanced beamsplitter. Conversely, fermionic anti-bunching can yield up to a twofold enhancement of coincidences compared to the baseline of distinguishable particles. As such, the emergence of dips or peaks in the HOM effect may appear indicative of the particles' bosonic/fermionic nature. Here, we demonstrate experimentally that the coincidence statistics of boson pairs can be seamlessly tuned from full suppression to enhancement by an appropriate choice of the observation basis. Our photonic setting leverages birefringent couplers6 to introduce differential dissipation in the photons' polarization. In contrast to previous work7–9, the mechanism underpinning this unusual behaviour does not act on individual phases accumulated by pairs of particles along specific paths, but instead allows them to jointly evade losses as indistinguishable photons are prevented from inhabiting orthogonal modes. Our findings reveal a new approach to harnessing non-Hermitian settings for the manipulation of multi-particle quantum states and as functional elements in quantum simulation. Polarization-dependent losses shape Hong–Ou–Mandel interference of photon pairs in birefringent waveguides. Seamless tunability of indistinguishable photon coincidences, all the way from enhancement to full suppression, is enabled by an appropriate choice of the observation basis. [ABSTRACT FROM AUTHOR]