Light quantum interference: anomalous phenomena discovered

(Nanowerk News) One of the cornerstones of quantum physics is Niels Bohr’s principle of complementarity, roughly speaking of the fact that objects can behave like particles or like waves. These two mutually exclusive descriptions are well illustrated in the iconic double slit experiment, in which particles impinge on a plate containing two slits. If the individual particle trajectories are not considered, we will observe a wave-like interference fringe when the particles collect after passing through the slit.

On the other hand, if the trajectory is observed, then the edges disappear and everything happens as if we were dealing with a particle-like ball in the classical world. As coined by physicist Richard Feynman, interference fringes stem from a lack of information about which path, so the fringes must disappear as soon as the experiment lets us know that each particle has taken one or the other path through the left or right slit.

Light is not immune to 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 amazing phenomenon: ie photo grouping.

In short, if there’s no way to tell photons apart 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 hitting each side of the semi-transparent mirror, which divides the incoming light into two possible paths related to the reflected and transmitted light.

Indeed, the famous Hong–Ou–Mandel effect tells us here that two outgoing photons always come out together on 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. Thus, logically, one would expect the clustering to become less clear-cut as soon as we can distinguish photons and trace back which path they have taken. This is exactly what is observed experimentally if two photons incident on a semi-transparent mirror have, for example, different polarizations or different colors: they behave like classical spheres and no longer cluster.

This interaction between photon grouping and dissimilarity is generally recognized to reflect a general rule: the aggregation should be maximal for photons that are completely indistinguishable and gradually decrease as the photons are made more distinguishable.

Against all odds, this common assumption was recently proven wrong by a team from the Center for Quantum Information and Communication (Ecole polytechnique de Bruxelles, Université libre de Bruxelles) led by Professor Nicolas Cerf, assisted by his PhD student, Benoît Seron, and postdoctoral fellow, Dr. Leonardo Novo, now a research staff at the Iberia International Nanotechnology Laboratory, Portugal.

They have considered a specific theoretical scenario in which seven photons strike a large interferometer and examined examples where all the photons cluster into the two output paths of the interferometer. Logical clustering must be strongest when all seven photons admit the same polarization because that makes them completely indistinguishable, meaning we get no information about their path in the interferometer.

Surprisingly enough, researchers have discovered the existence of several instances where the grouping of photons is substantially amplified – rather than weakened – by making the photons partially distinguishable through well-chosen polarization patterns.

The Belgian team exploited the link between the physics of quantum interference and a permanent mathematical theory. By leveraging the new conjecture disproved on matrix permanents, they were able to prove that it is possible to further improve the grouping of photons by fine-tuning the polarization of the photons.

As well as being of interest to the basic physics of photon interference, this anomalous clustering phenomenon should have implications for quantum photonics technology, which has shown rapid progress over the past few years.

Experiments aimed at building an optical quantum computer have reached an unprecedented level of control, in which multiple photons can be created, interfered through complex optical circuits, and counted with photon number-dividing detectors. Therefore, understanding the intricacies of photon grouping, which is related to the quantum bosonic nature of photons, is a significant step in this perspective.

The research has been reported in Nature Photonics (“Boson binding is not maximized by indistinguishable particles”).