One of the cornerstones of quantum physics is Niels Bohr's complementarity principle, roughly speaking the fact that objects may behave either like particles or like waves. These two mutually exclusive descrip­tions are well illustrated in the iconic double-slit experiment, where particles are impinging on a plate containing two slits. If the trajectory of each particle is not watched, one observes wave-like interference fringes when collecting the particles after going through the slits. On the contrary, if the trajec­tories are watched, then the fringes disappear and everything happens as if we were dealing with particle-like balls in a classical world. The interference fringes originate from the absence of which-path information, so that the fringes must necessarily vanish as soon as the experiment allows us to learn that each particle has taken one or the other path through the left or right slit.

Light does not escape this duality: it can either be described as an electro­magnetic wave or photons. This comes with another remarkable phenomenon: that of photon bunching. Loosely speaking, if there is no way to distinguish photons and know which path they follow in a quantum inter­ference experiment, then they tend to stick together. This behavior can already be observed with two photons impinging each one on a side of a half-transparent mirror, which splits the incoming light into two possible paths associated with reflected and transmitted light. Indeed, the cele­brated Hong–Ou–Mandel effect tells us here that the two outgoing photons always exit together on the same side of the mirror, which is a consequence of a wave-like inter­ference between their paths.

This bunching effect cannot be understood in a classical worldview where we think of photons as classical balls, each one taking a well-defined path. Thus, logically, it is expected that bunching becomes less pronounced as soon as we are able to distinguish the photons and trace back which paths they have taken. This is precisely what one observes experimentally if the two incident photons on the half-transparent mirror have, for example, distinct polarization or different colors: they behave as classical balls and do not bunch any more. This interplay between photon bunching and distinguisha­bility is commonly admitted to reflect a general rule: bunching must be maximum for fully indis­tinguishable photons and gradually decline when photons are made increasingly dis­tinguishable.

Against all odds, this common assumption has recently been proven wrong by a team from the Center for Quantum Information and Communi­cation, Ecole poly­technique de Bruxelles, Université libre de Bruxelles, led by Nicolas Cerf, assisted by his PhD student, Benoît Seron, and his postdoc, Leonardo Novo. They have considered a specific theoretical scenario where seven photons impinge on a large inter­ferometer and probed the instances where all photons bunch into two output paths of the inter­ferometer. Bunching should logically be the strongest when all seven photons admit the same polari­zation since it makes them fully indis­tinguishable, meaning that we get no information on their paths in the inter­ferometer. Quite surprisingly, the researchers have discovered the existence of some instances where photon bunching is sub­stantially strengthened – instead of weakened – by making photons partially distinguishable via a well-chosen polari­zation pattern.

The Belgian team took advantage of a connection between the physics of quantum inter­ferences and the mathematical theory of permanents. By leveraging a newly disproved conjecture on matrix permanents, they could prove that it is possible to further enhance photon bunching by fine-tuning the polarization of the photons. Aside from being intriguing for the fundamental physics of photon interference, this anomalous bunching phenomenon should have impli­cations for quantum photonic technologies, which have shown fast progress over the recent years. Experiments aimed at building an optical quantum computer have reached an unprecedented level of control, where many photons can be created, interfered via complex optical circuits, and counted with photon-number resolving detectors. Understanding the subtleties of photon bunching, which is linked to the quantum bosonic nature of photons, is therefore a signi­ficant step in this perspective. (Source: ULB)

Reference: B. Seron et al.: Boson bunching is not maximized by indistinguishable particles, Nat. Phot., online 15 June 2023; DOI: 10.1038/s41566-023-01213-0

Link: Centre for Quantum Information and Communication, Ecole polytechnique de Bruxelles, Université libre de Bruxelles, Brussels, Belgium