Physicists Achieve Breakthrough in Controlling Photonic Bose-Einstein Condensate

In a recent study published in Nature Communications, researchers from the University of Bonn demonstrated that photon Bose-Einstein condensates adhere to a fundamental physics theorem. This discovery enables the measurement of properties of photon Bose-Einstein condensates, which are typically challenging to access.

Physicists Achieve Breakthrough in Controlling Photonic Bose-Einstein Condensate
Artist view - of a photonic Bose-Einstein condensate (yellow) in a bath of dye molecules (red) that has been perturbated by an external light source (white flash). Image Credit: A. Erglis/Albert-Ludwigs University of Freiburg.

Many atoms can behave like a single “super particle" if they are confined in a small volume and cooled to an extremely low temperature. This is also called a quantum gas or Bose-Einstein condensate by physicists. Photons condense according to a similar principle and can be cooled utilizing dye molecules. These dye molecules swallow the “hot” light particles and spit them out at the correct temperature.

In our experiments we filled a tiny container with a dye solution. The walls of the container were highly reflective.

Dr. Julian Schmitt, Institute of Applied Physics, University of Bonn

Using a laser, the researchers stimulated the dye molecules. As a result, photons were created, which oscillated between the reflective surfaces. As the light particles repeatedly collided with the dye molecules, they cooled and condensed into a quantum gas. 

Super Photons Flicker Like a Candle

This process persists, with the super photons' particles repeatedly colliding with the dye molecules, being absorbed before being ejected again. As a result, the quantum gas flickers like a candle (as it contains varying amounts of photons over time).

We used this flickering to investigate whether an important theorem of physics is valid in a quantum gas system.

Dr. Julian Schmitt, Institute of Applied Physics, University of Bonn

An easy analogy can be used to explain this so-called "regression theorem": Assume that the super photon is a campfire that sporadically flares up violently at random. The fire gradually goes out of control and returns to its initial state after it flares up brightly.

Blowing air into the embers can also purposefully cause the fire to flare up.

The regression theorem states that after that, the fire will extinguish itself in the same manner as if the flare-up had happened at random. This indicates that it behaves in the same manner in response to the disturbance as it does when it fluctuates naturally.

Blowing Air into a Photon Fire

Schmitt, who is also a member of the transdisciplinary research area (TRA) “Building Blocks of Matter” and the “Matter and Light for Quantum Computing” Cluster of Excellence at the University of Bonn, added, “We wanted to find out whether this behavior also applies to quantum gases.

For this purpose, the researchers initially measured the flickering of the super photons to assess the statistical fluctuations. Subsequently, metaphorically speaking, they intensified the effect by briefly applying another laser to the super photon. This perturbation led to a brief flare-up before gradually returning to its initial state.

Schmit continued, “We were able to observe that the response to this gentle perturbation follows precisely the same dynamics as the random fluctuations without a perturbation. In this way, we were able to demonstrate for the first time that this theorem also applies to exotic forms of matter as quantum gases.

Interestingly, this phenomenon holds even for strong perturbations. Typically, systems exhibit different responses to stronger perturbations than weaker ones–an extreme example is a layer of ice suddenly breaking under excessive load.

Schmit adds, “This is called nonlinear behavior. However, the theorem remains valid in these cases, as we have now been able to demonstrate together with our colleagues from the University of Antwerp.”

The findings are relevant to fundamental research involving photonic quantum gases, as their brightness fluctuations are often unpredictable. Understanding how the super photon responds to controlled perturbations provides a more accessible means of investigation.

Schmit concluded, “This allows us to learn about unknown properties under very controlled conditions. It will enable us, for example, to find out how novel photonic materials consisting of many super photons behave at their core.”

The Institute of Applied Physics at the University of Bonn, the University of Antwerp (Belgium), and the University of Freiburg contributed to the study. The project received support from the German Research Foundation (DFG), the European Union (ERC Starting Grant), the German Aerospace Centre (DLR), and the Belgian funding agency FWO Flanders.

Journal Reference:

Sazhin, A., et al. (2024) Observation of nonlinear response and Onsager regression in a photon Bose-Einstein condensate. Nature Communications.

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