Boson bunching is not maximized by indistinguishable particles

TL;DR

This paper demonstrates that photon bunching can be enhanced by partial distinguishability, challenging the traditional view that indistinguishability maximizes boson bunching, with implications for quantum optics experiments.

Contribution

It introduces optical circuits where partial distinguishability boosts photon bunching, countering the conventional assumption linking indistinguishability to maximum bunching.

Findings

Photon bunching can be increased by partial distinguishability.

The effect is observable in a 7-photon interferometric setup.

Challenges existing understanding of multiparticle interference.

Abstract

Boson bunching is amongst the most remarkable features of quantum physics. A celebrated example in optics is the Hong-Ou-Mandel effect, where the bunching of two photons arises from a destructive quantum interference between the trajectories where they both either cross a beam splitter or are reflected. This effect takes its roots in the indistinguishability of identical photons. Hence, it is generally admitted -- and experimentally verified -- that bunching vanishes as soon as photons can be distinguished, e.g., when they occupy distinct time bins or have different polarizations. Here we disproof this alleged straightforward link between indistinguishability and bunching by exploiting a recent finding in the theory of matrix permanents. We exhibit a family of optical circuits where the bunching of photons into two modes can be significantly boosted by making them partially distinguishable via an appropriate polarization pattern. This boosting effect is already visible in a 7-photon interferometric process, making the observation of this phenomenon within reach of current photonic technology. This unexpected behavior questions our understanding of multiparticle interference in the grey zone between indistinguishable bosons and classical particles.