Physicists from the Institute of Applied Physics (IAP) at the University of Bonn, in cooperation with colleagues at the University of Kaiserslautern-Landau (RPTU), have created a one-dimensional gas out of light particles. The work enables the testing of theories about the transition to this state of matter for the first time. “To create these types of gases, we need to concentrate lots of photons in a confined space and cool them simultaneously,” said Frank Vewinger from the IAP. In their experiment, the researchers filled a tiny container with a dye solution and excited it using a laser. The resulting photons bounced back and forth between the reflective walls of the container. Whenever they collided with a dye molecule, they were cooled until the photon gas ultimately condensed. The polymers applied to the reflective surface trap the photon gas in a parabola of light. The narrower this parabola is, the more one-dimensionally the gas behaves. Courtesy of IAP/University of Bonn. The dimensionality of the gas can be influenced by modifying the surface of the reflective surfaces. A high-resolution structuring method was adapted so that it could be applied to the reflective surfaces of the photon container for this experiment. “We were able to apply a transparent polymer to the reflective surfaces to create microscopically small protrusions,” said Julian Schulz from the RPTU. “These protrusions allow us to trap the photons in one or two dimensions and condense them.” In two dimensions, there is a precise temperature limit at which condensation occurs — similar to how water freezes at precisely zero degrees Celsius. Physicists call this a phase transition. “However, things are a little different when we create a one-dimensional gas instead of a two-dimensional one,” says Vewinger. “So-called thermal fluctuations take place in photon gases but they are so small in two dimensions that they have no real impact. However, in one dimension these fluctuations can — figuratively speaking — make big waves.” These fluctuations destroy the order of one-dimensional systems so that different regions within the gas no longer behave the same. As a result, the phase transition, which is still precisely defined in two dimensions, becomes increasingly “smeared out” the more one-dimensional the system becomes. However, its properties are still governed by quantum physics, as in the case of two-dimensional gases, and these types of gas are called degenerate quantum gases. It is as if water becomes a form of icy water at low temperatures without ever completely freezing when cooled. “We have now been able to investigate this behavior at the transition from a two-dimensional to a one-dimensional photon gas for the first time,” said Vewinger. The research groups were able to demonstrate that one-dimensional photon gases do not actually have a precise condensation point. By making tiny changes to the polymer structures, it will now be possible to investigate phenomena that occur at the transition between different dimensionalities in great detail. While the work is in its early stages, the team believes it could open applications for quantum optical effects. The research was published in Nature Physics (www.doi.org/10.1038/s41567-024-02641-7).