An artist’s view of a photonic Bose-Einstein condensate (yellow) in a bath of dye molecules (red) that has been disrupted by an external light source (white flash). Credit: A. Erglis/Albert-Ludwigs University of Freiburg
Scientists from the University of Bonn have shown that super photons, or photon Bose-Einstein condensates correspond to fundamental physical theorems and provide insight into properties that are often difficult to observe.
Under the right conditions, thousands of particles of light can combine into a type of “superphoton”. Physicists call such a state a photon Bose-Einstein condensate. Scientists from the University of Bonn have now shown that this exotic quantum state obeys a fundamental theorem of physics. This finding now makes it possible to measure properties of photonic Bose-Einstein condensates, which are usually difficult to access. The study was published June 3 in the journal The nature of communication.
If many atoms are cooled to a very low temperature in a small volume, they can become indistinguishable and behave as a single “superparticle”. Physicists also call it a Bose-Einstein condensate or a quantum gas. Photons condense on a similar principle and can be cooled using dye molecules. These molecules act like little refrigerators, swallowing “hot” light particles before spitting them out again at the right temperature.
Experimenting with super photons in quantum gases
“During our experiments, we filled a small container with a dye solution,” explains Dr. Julian Schmitt from the Institute of 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 the 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 and are absorbed before being spat out again. Therefore, the quantum gas sometimes contains more and sometimes fewer photons, so it flickers like a candle. “We used this flicker 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: Suppose that a superphoton is a campfire that sometimes accidentally flares up very brightly. After the fire burns particularly brightly, the flames slowly die down and the fire returns to its original state. Interestingly, a fire can also be caused by intentional ignition by blowing air into the embers. Simply put, the regression theorem predicts that a fire will continue to burn in the same way as if it had started randomly. This means that it reacts to a disturbance in exactly the same way as it fluctuates without any disturbance.
Blowing air into the photon fire
“We wanted to find out 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 “Matter and Light for Quantum Computing” cluster. Excellence at the University of Bonn. To do this, the researchers first measured the flickering of superphotons to quantify the statistical fluctuations. They then – figuratively speaking – blew air into the fire by briefly firing another superphoton laser. This malfunction caused him to flare up briefly before slowly returning to his original state.
Demonstration of nonlinear behavior in quantum systems
“We were able to observe that the response to this subtle perturbation has exactly the same dynamics as random fluctuations without the perturbation,” 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 is also true for strong perturbations. Systems usually respond differently to stronger disturbances than to weaker ones – an extreme example is a sheet of ice that suddenly breaks when the stress on it is too great. “This is called non-linear behavior,” says Schmitt. “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.”
Implications for research in photonic quantum gases
The findings have huge implications for fundamental research with photonic quantum gases, as one often does not know exactly how they will flicker in their brightness. It is much easier to determine how a superphoton responds to a controlled perturbation. “This allows us to learn about unknown properties under very controlled conditions,” explains Schmitt. “For example, it will allow us to find out how new photonic materials consisting of many superphotons behave at their core.”
Reference: “Observation of nonlinear response and Onsager regression in a photonic Bose-Einstein condensate” by Alexander Sazhin, Vladimir N. Gladilin, Andris Erglis, Göran Hellmann, Frank Vewinger, Martin Weitz, Michiel Wouters, and Julian Schmitt, 03 Jun 2024 The nature of communication.
DOI: 10.1038/s41467-024-49064-9
The Institute of Applied Physics at 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 funding agency FWO Flanders.