Three researchers from the Free University of Brussels (Belgium) have discovered a controversial aspect of photon interference physics. In an article published this month Nature Photonics, proposed a fictitious experiment that completely contradicted the general knowledge about the so-called photon grouping property. Observation of this anomalous aggregation effect appears to be within the reach of current photonics technologies and, if achieved, will greatly impact our understanding of multiparticle quantum interference.

One of the cornerstones of quantum physics is Niels Bohr’s principle of complementarity, which states that, roughly speaking, objects can behave like particles or waves. These two mutually exclusive explanations are well illustrated in the iconic double-slit experiment, in which particles collide with a plate containing two slits. Without observing the trajectory of each particle, wave-like interference fringes can be observed as particles collect after passing through the slits. But if you observe the orbits, the lines disappear and everything becomes as if we were dealing with particle-like spheres in the classical world.

According to physicist Richard Feynman, interference fringes are caused by a lack of information about which path, so as soon as experiment allows us to know that each particle follows one or the other path through the left or right slit, the fringes must necessarily disappear. .

Light cannot escape this duality: it can either be described as an electromagnetic wave or understood to be composed of massless particles, namely photons, traveling at the speed of light. This is linked to another remarkable phenomenon: the grouping of photons. Roughly speaking, if there is no way to distinguish photons in a quantum interference experiment and know which way they go, then they tend to stick together.

This behavior can already be observed when two photons collide with the edge of a translucent mirror, which splits the incident light into two possible paths associated with the reflected and transmitted light. Indeed, the well-known Hong-Ou-Mandel effect tells us here that two photons going out always come together from the same side of the mirror, which is a result of wave-like interference between their paths.

This clustering effect cannot be understood in the classical worldview, where we think of photons as classical spheres, each with a well-defined path. Therefore, it is reasonable to expect the grouping to become less obvious once you distinguish the photons and watch which path they take. This is what can be observed experimentally if two photons falling into a translucent mirror, for example, have different polarizations or different colors: they behave like classical balls and no longer collide. It is generally accepted that this interaction between photon grouping and resolution reflects a general rule: for photons that are completely indistinguishable, the grouping should be maximum and gradually decrease as the photons become more distinct.

However, the fallacy of this common assumption was recently discovered by Professor Nicolas Cerf of the Center for Quantum Information and Communication at the Free University of Brussels Ecole Polytechnique de Bruxelles and Ph.D. student Benoit Seron and his postdoctoral fellow Dr. Leonardo Novo.

They considered a particular theoretical scenario in which seven photons hit a large interferometer and investigated cases where all photons were collected in the two exit paths of the interferometer. Logically, grouping should be strongest when all seven photons are allowed the same polarization, since this makes them completely indistinguishable, meaning we get no information about their paths in the interferometer. Surprisingly, the researchers found that there are some cases where photon bunching increases significantly rather than weakens because the photons are partially separated by a well-chosen polarization pattern.

The Belgian team took advantage of the connection between quantum interference physics and the mathematical theory of persistence. Using the recently refuted permanent matrix hypothesis, they could prove that it is possible to further improve photon bunching by fine-tuning the polarization of photons. Besides the intrigue of the fundamental physics of photon interference, this anomalous grouping phenomenon should also have implications for quantum photon technologies, which have been rapidly advancing in recent years.

Experiments aimed at creating an optical quantum computer have reached an unprecedented level of control, in which many photons can be created, interfered with via complex optical circuits, and counted using detectors that distinguish the number of photons. Therefore, understanding the intricacies of photon grouping that relates to the quantum bosonic nature of photons is an important step in this perspective. Source