Artist’s rendering of a photonic Bose-Einstein condensate (yellow) in a bath of dye molecules (red) disturbed by an external light source (white flash). Credit: A. Erglis/Albert-Ludwigs University of Freiburg
Researchers from the University of Bonn have shown that superphotons, or photon Bose-Einstein condensates satisfy fundamental physics theorems and enable insights into properties that are often difficult to observe.
Under suitable conditions, thousands of light particles can merge into a kind of ‘super photon’. Physicists call such a state a photon-Bose-Einstein condensate. Researchers from the University of Bonn have now shown that this exotic quantum state satisfies a fundamental theorem of physics. This finding now makes it possible to measure properties of photon Bose-Einstein condensates, which are typically difficult to access. The study was published June 3 in the journal Nature communication.
If many atoms are cooled to a very low temperature, confined in a small volume, they can become indistinguishable from each other and behave as a single ‘superparticle’. Physicists also call this a Bose-Einstein condensate or quantum gas. Photons condense based on a similar principle and can be cooled using dye molecules. These molecules work like small refrigerators and swallow the ‘hot’ light particles before spitting them out at the right temperature.
Experimenting with superphotons in quantum gases
“In our experiments we filled a small container with a dye solution,” explains Dr. Julian Schmitt from the Institute for Applied Physics at the University of Bonn. “The walls of the container were highly reflective.” The researchers then excited the dye molecules with a laser. This produced photons that bounced back and forth between the reflective surfaces. As the light particles repeatedly collided with dye molecules, they cooled and eventually condensed into a quantum gas.
However, this process continues afterwards and the superphoton particles repeatedly collide with the dye molecules, being swallowed up before being spit out again. That is why the quantum gas sometimes contains more and sometimes fewer photons, causing it to flicker like a candle. “We used this flickering to investigate whether an important physics theorem holds in a quantum gas system,” says Schmitt.
Understanding the regression theorem in quantum gases
This so-called ‘regression theorem’ can be illustrated with a simple analogy: let’s assume that the superphoton is a campfire that sometimes randomly flares up very strongly. After the fire has flamed particularly brightly, the flames slowly extinguish and the fire returns to its original state. Interestingly, you can also deliberately fan the fire by blowing air into the embers. In simple terms, the regression theorem predicts that the fire will then continue to burn out in the same way as if the flare-up had occurred randomly. This means that it responds to the disturbance in exactly the same way as it fluctuates on its own, without any disturbance.
Blowing air into a photon fire
“We wanted to know whether this behavior also applies to quantum gases,” explains Schmitt, who is also a member of the transdisciplinary research area (TRA) “Building Blocks of Matter” and the cluster “Matter and Light for Quantum Computing”. Excellence at the University of Bonn. For this purpose, the researchers first measured the flicker of the superphotons to quantify the statistical fluctuations. They then blew air into the fire – figuratively speaking – by briefly firing another laser at the superphoton. This disturbance caused it to flare up briefly before slowly returning to its original state.
Demonstrating nonlinear behavior in quantum systems
“We have been able to observe that the response to this gentle disturbance follows exactly the same dynamics as the random fluctuations without disturbance,” says the physicist. “In this way we were able to demonstrate for the first time that this theorem also applies to exotic forms of matter such as quantum gases.” Interestingly, this also applies to strong disturbances. Systems generally respond differently to stronger disturbances than to weaker ones. An extreme example is an ice layer that suddenly breaks when the load placed on it becomes too heavy. “This is called non-linear behavior,” says Schmitt. “In these cases, however, the statement remains valid, as we have now been able to demonstrate together with our colleagues from the University of Antwerp.”
Implications for research into photonic quantum gases
The findings are of great importance for fundamental research with photonic quantum gases, because it is often not known exactly how they will flicker in brightness. It is much easier to determine how the superphoton responds to a controlled perturbation. “This allows us to gain knowledge about unknown properties under highly controlled conditions,” Schmitt explains. “For example, it will allow us to find out how new photonic materials, consisting of many superphotons, behave in their core.”
Reference: “Observation of Nonlinear Response and Onsager Regression in a Photon Bose-Einstein Condensate” by Alexander Sazhin, Vladimir N. Gladilin, Andris Erglis, Göran Hellmann, Frank Vewinger, Martin Weitz, Michiel Wouters and Julian Schmitt, 3 June 2024, Nature communication.
DOI: 10.1038/s41467-024-49064-9
The Institute for Applied Physics of the University of Bonn, the University of Antwerp (Belgium) and the University of Freiburg participated in the study. The project was supported by the German Research Foundation (DFG), the European Union (ERC Starting Grant), the German Aerospace Center (DLR) and the Belgian financing company FWO-Vlaanderen.