Quantum interference of light: an anomalous phenomenon found

Three researchers from Université Libre de Bruxelles in Belgium have discovered a counterintuitive side to the physics of photon interference. In an article published this month in Nature photonics, they proposed a thought experiment that completely contradicts common knowledge about the so-called collecting property of photons. Observing this anomalous grouping effect seems to be within the reach of current photonic technologies, and if achieved, would severely impact our understanding of quantum multiparticle interferences.

One of the cornerstones of quantum physics is Niels Bohr’s principle of complementarity, which states, roughly speaking, that things may behave either like particles or like waves. These two conflicting descriptions are well illustrated in the iconic double-slit experiment, in which particles strike a plate containing two slits. If the trajectory of each particle is not monitored, one notices wave-like interference fringes when particles are collected after passing through the slits. But if the trajectories are monitored, the limbs disappear and everything happens as if we were dealing with particle-like spheres in a classical universe.

As physicist Richard Feynman coined it, interference fringes arise from the lack of “which path” information, so the fringes must necessarily disappear once experience lets us know that each particle has taken one or another path through the right or left slit.

Light does not escape this duality: it can either be described as an electromagnetic wave or it can be understood as consisting of massless particles moving at the speed of light, that is, photons. This comes with another fascinating phenomenon: the phenomenon of photon aggregation. Loosely speaking, if there is no way to tell the photons apart and know which path they follow in a quantum interference experiment, they tend to stick together.

This behavior can already be observed with two photons colliding each other on the side of a translucent mirror, which splits the incoming light into two possible paths related to reflected and transmitted light. In fact, the famous Hung Ou Mandel effect here tells us that the two external photons always exit together on the same side of the mirror, which is the result of wave-like interference between their paths.

This grouping effect cannot be understood in the classical worldview where we think of photons as classical balls, each one taking a well-defined path. Thus, logically speaking, clustering is expected to become less clear once we can distinguish photons and track the paths they have taken. This is exactly what one observes experimentally if the two photons incident on the translucent mirror, for example, have distinct polarizations or different colours: they behave as classical spheres and no longer clump together. This interaction between aggregation of photons and differentiability is generally recognized to reflect a general rule: aggregation should be maximal for photons that are completely indistinguishable and gradually decrease as photons are increasingly distinguished.

Against all odds, this common assumption has recently been proven wrong by a team from the Center for Quantum Information and Communication at the Brussels Polytechnic, Universitaire Libre de Brussels led by Professor Nicolas Cerf, helping him obtain his Ph.D. Student, Benoit Cerone, and postdoctoral researcher, Dr. Leonardo Novo, are now working researchers at the Iberian International Laboratory for Nanotechnology, Portugal.

They studied a specific theoretical scenario where seven photons collide with a large interferometer and probed cases where all the photons converged into two output paths of the interferometer. The grouping should logically be strongest when all seven photons admit the same polarization because it makes them completely indistinguishable, which means we don’t get information about their trajectories in the interferometer. Surprisingly, the researchers found that there are some cases in which photon aggregation is greatly strengthened – rather than weakened – by making the photons partly distinguishable via a well-chosen polarization pattern.

The Belgian team took advantage of a link between the physics of quantum interference and the mathematical theory of time. By making use of a recently refuted conjecture regarding matrix permanents, they can demonstrate that it is possible to further enhance photon scattering by tuning the photons’ polarization. Apart from being interesting for the fundamental physics of photon interference, this anomalous grouping phenomenon should have implications for quantum photonic technologies, which have shown rapid progress over recent years.

Experiments aimed at building an optical quantum computer have reached an unprecedented level of control, where many photons can be created, interfering through complex optical circuits, and counted with photon number detectors. Therefore understanding the subtleties of photon aggregation, which is related to the quantum bosonic nature of photons, is an important step in this perspective.