Scientists have discovered an unexpected quantum interference anomaly

Anomalous grouping effect in which all photons converge into two output beams. Credit: Ursula Cardenas Mamani

In a recently published paper in Nature photonicsScientists from the Center for Quantum Information and Communication – Polytechnic Brussels Vrije University of Brussels report the discovery of an unexpected counterexample that challenges the conventional understanding of quantum. Photon compilation.

Niels Bohr’s principle of complementarity, a fundamental concept in quantum physics, basically states that objects can exhibit either particle-like or wave-like behaviour. These two contrasting descriptions are well illustrated in the famous double-slit experiment, in which particles strike a plate containing two slits.

If the trajectory of each particle is not monitored, a wave-like interference fringe can be observed when the particles are collected after passing through the slits. On the contrary, if the trajectories are observed, the limbs disappear and everything happens as if we were dealing with particle-like spheres in a classical world.

As the physicist Richard Feynman formulated it, interference fringes arise from absence any track information, so the edges must necessarily disappear once the experiment lets us know that each particle has taken one path or another through the left or right slit.

Light does not escape this duality: it can be described as an electromagnetic wave or it can be understood as consisting of massless particles traveling at the speed of light, which are photons. And this comes with another notable phenomenon: a phenomenon photon assembly. In general, if there is no way to distinguish photons and know which path they follow in a quantum interference experiment, they tend to stick together.

This behavior can already be observed when two photons collide each other next to a translucent mirror, splitting the incoming light into two possible paths related to reflected and transmitted light. In fact, the famous Hong-O-Mandel effect here tells us that the two exiting photons always exit together on the same side of the mirror, which is the result of wave-like interference between their paths.

This clustering effect cannot be understood from the classical worldview where we think of photons as classical balls, each one taking a well-defined path. Thus, it would logically be expected that clustering would become less obvious once we could distinguish photons and trace the paths they had taken.

This is exactly what we observe experimentally if, for example, two photons falling on a semi-transparent mirror have distinct polarizations or different colors: they behave like classical balls and no longer aggregate. It is generally accepted that this interaction between photon aggregation and discriminability reflects a general rule: aggregation should reach a maximum for photons that are completely indistinguishable, and then gradually decrease as photons become increasingly indistinguishable.

Against all odds, this common assumption has recently been proven wrong by a team from the Center for Quantum Information and Communication (Brussels Polytechnic School of the Université Libre de Bruxelles) led by Professor Nicolas Cerf, with the assistance of Ph.D. student, Benoit Ceron, and postdoctoral researcher, Dr. Leonardo Novo, who is now a researcher at the Iberian International Nanotechnology Laboratory, Portugal.

They considered a specific theoretical scenario in which seven photons collide with a large interferometer, and studied cases in which all the photons converge into two paths to the interferometer’s output. It stands to reason that the summation is strongest when all seven photons admit the same polarization, because it makes them completely indistinguishable, meaning we get no information about their paths in the interferometer. Surprisingly, the researchers discovered that there are some cases in which photon gathering 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 relationship between quantum interference physics and the mathematical theory of constants. Taking advantage of the recently disproven conjecture about the permanent array, they can demonstrate that it is possible to further enhance photon collection by tuning the polarization of the photons.

Apart from being interesting for the basic physics of photon interference, this abnormal aggregation phenomenon It should have implications for quantum photonic technologies, which have shown rapid progress over recent years.

Experiments aiming to build an optical quantum computer have reached an unprecedented level of control, where many photons can be created, interfered through complex optical circuits, and counted by photon number crunching detectors. Understanding the finer details of photon aggregation, which is related to the quantum bosonic nature of photons, is an important step in this perspective.

Reference: “Boson aggregation is not maximized by indistinguishable particles” by Benoit Ceron, Leonardo Novo and Nicholas J. Cerf, 15 June 2023, Nature photonics.
doi: 10.1038/s41566-023-01213-0