Quantum Interference of Light: An Anomalous Phenomenon Discovered

Three researchers from the Université libre de Bruxelles, Belgium, have uncovered a counterintuitive aspect of the physics of photon interference. In an article published this month in Photonics of nature, they proposed a thought experiment that completely contradicts conventional wisdom about the so-called bunching property of photons. The observation of this anomalous group effect appears to be within the reach of today’s photonic technologies and, if achieved, would greatly impact our understanding of many-particle quantum perturbations.

One of the cornerstones of quantum physics is Niels Bohr’s principle of complementarity, which, roughly speaking, states that objects can behave either like particles or like waves. These two mutually exclusive descriptions are well illustrated in the iconic double-slit experiment, where particles are impinged on a plate containing two slits. If the trajectory of each particle is not observed, wavy interference fringes are observed as the particles collect after passing through the slits. But if the trajectories are observed, then the fringes disappear and everything happens as if we were dealing with particle-like balls in a classical world.

As coined by the physicist Richard Feynman, interference fringes arise from the lack of information about “which way”, so the fringes must necessarily disappear as soon as the experiment allows us to learn that each particle took one or the other path through left or right slot

Light does not escape this duality: it can be described as an electromagnetic wave, or it can be understood as consisting of massless particles moving at the speed of light, namely photons. This comes with another remarkable phenomenon: photon bunching. Loosely defined, if there is no way to distinguish the photons and know which path they follow in a quantum interference experiment, then they tend to clump together.

This behavior can now be observed with two photons colliding with each on one side of a translucent mirror, which splits the incoming light into two possible paths associated with reflected and transmitted light. Indeed, the famous Hong-Ou-Mandel effect tells us here that the two outgoing photons always exit together from the same side of the mirror, which is a consequence of wave-like interference between their paths.

This clustering effect cannot be understood in a classical worldview, where we think of photons as classical balls, each taking a well-defined path. Therefore, logically, the clustering is expected to become less pronounced as soon as we can distinguish the photons and trace what paths they took. This is exactly what is observed experimentally if the two incident photons on the semi-transparent mirror have, for example, different polarization or different colors: they behave like classical balls and no longer add up. This interaction between photon clustering and discriminability is generally taken to reflect a general rule: clustering should be maximal for completely indistinguishable photons and gradually decrease as photons become more distinguishable.

Against all odds, this general assumption was recently proven wrong by a team from the Center for Quantum Information and Communication at the Ecole polytechnique de Bruxelles, Université libre de Bruxelles, led by Professor Nicolas Cerf, assisted by his Ph.D. student, Benoit Seron, and his postdoc, Dr. Leonardo Novo, now a staff researcher at the International Iberian Nanotechnology Laboratory, Portugal.

They considered a specific theoretical scenario in which seven photons collide with a large interferometer and investigated the cases where all photons converge into two exit paths of the interferometer. The clustering should logically be strongest when all seven photons assume the same polarization, as this makes them completely indistinguishable, meaning we get no information about their paths in the interferometer. Rather surprisingly, the researchers have discovered the existence of some cases where photon bunching is greatly enhanced — rather than weakened — by making the photons partially distinguishable through a well-chosen polarization pattern.

The Belgian team took advantage of the connection between the physics of quantum perturbations and the mathematical theory of permanents. By using a recently disproved hypothesis of matrix permanents, they could demonstrate that it is possible to further improve photon bunching by fine-tuning the polarization of the photons. In addition to being intriguing for the fundamental physics of photon interference, this anomalous clustering phenomenon should have implications for quantum photon technologies, which have shown rapid progress in recent years.

Experiments aimed at building an optical quantum computer have reached an unprecedented level of control where many photons can be created, interfered with by complex optical circuits and counted with photon-resolving detectors. Therefore, understanding the intricacies of photon bunching, which is related to the quantum boson nature of photons, is a significant step in this perspective.