Researchers generate one-dimensional gas from light: Joint experiment by the University of Bonn and RPTU
Physicists from the University of Bonn and the Rhineland-Palatinate University of Technology Kaiserslautern-Landau (RPTU) have created a one-dimensional gas from light.This allowed them to test theoretical predictions made for the transition to this exotic state of matter for the first time.The method they used in their experiment can be used to study quantum effects.The results have been published in the journal Nature Physics.
Suppose you are standing by a swimming pool and have the idea of filling it with even more water. So you grab the garden hose and create a jet that falls in a high arc onto the surface of the pool. Where the jet hits, the water level will rise briefly. However, this is only minimal because the falling water is quickly distributed over the extended surface.
However, the situation is different if you fill a gutter with your garden hose: This creates a wave of water at the point where you point the hose. The walls of the gutter now ensure that the filled water no longer flows freely along a surface, but can only spread in the direction of the gutter. The amplitude of the wave increases the narrower the channel is, i.e. the more ‘one-dimensional’ it becomes.
The physicists at the Institute of Applied Physics (IAP) at the University of Bonn, in collaboration with colleagues from RPTU Kaiserlautern-Landau, have now investigated whether similar dimensionality effects also occur for gases made of light particles. ‘In order to produce such gases, we have to concentrate many photons in a small space and cool them down at the same time,’ explains Dr Frank Vewinger from the IAP, who is also a member of the Transdisciplinary Research Unit “Matter” at the University of Bonn.
Microscopically small rain gutters
In their experiment, the researchers filled a tiny container with a dye solution, which they excited with a laser beam. The photons generated were then reflected back and forth between the mirrored walls of the container. Whenever they collided with a dye molecule, they were cooled down until the photon gas finally condensed.
The dimensionality of the gas can be influenced by modifying the mirror surfaces. The IAP researchers collaborated with the working group led by Prof Dr Georg von Freymann from the RPTU Kaiserslautern-Landau. For the experiments, a high-resolution structuring method was specially adapted to the reflective surfaces of the photon container. ‘This enabled us to apply microscopically small elevations made of a transparent polymer to the mirrors,’ explains Julian Schulz from RPTU. ‘These allow us to capture and condense the photons in either one or two dimensions.’
‘So these polymers act as a kind of gutter, only for light,’ says Kirankumar Karkihalli Umesh, first author of the study. ‘The narrower we make this channel, the more one-dimensional the gas behaves.’
Thermal fluctuations soften the condensation point
In two dimensions, there is an exact temperature limit at which condensation takes place - very similar to water, which freezes at exactly zero degrees Celsius. In physics, this is referred to as a phase transition. ‘But it should be different if we create a one-dimensional gas instead of a two-dimensional one,’ says Vewinger. ‘There are so-called thermal fluctuations in photon gases. These are so small in two dimensions that they don't cause any further disturbance. In one dimension, on the other hand, they create large waves, figuratively speaking.’
These fluctuations destroy the order in one-dimensional systems so that different areas of the gas no longer behave in the same way. This ensures that the phase transition, which is still precisely defined in two dimensions, ‘smears out’ more and more the more one-dimensional the system becomes. Nevertheless, as in the two-dimensional case, its properties are determined by quantum physics - such gases are also known as quantum degenerate. It is roughly the same as water turning into a kind of ice water at low temperatures, but never freezing completely. ‘We have now succeeded for the first time in investigating this behaviour at the transition from a two-dimensional to a one-dimensional photon gas,’ explains Vewinger.
The research groups were able to prove that one-dimensional photon gases do not actually have a sharp condensation point. By making tiny changes to the polymer structures, phenomena based on the transition between different dimensionalities can now be investigated in detail. At the moment, this is basic research. However, it is possible that new applications for quantum optical effects will emerge.
Participating institutions and funding:
The IAP at the University of Bonn, the Fraunhofer Institute for Industrial Mathematics (ITWM) in Kaiserslautern and the Rhineland-Palatinate Technical University of Kaiserslautern-Landau were involved in the study. The study was funded by the European Research Council (ERC) of the EU and the German Research Foundation (DFG) as part of the Collaborative Research Centre SFB TRR 185.
Publication: Kirankumar Karkihalli Umesh, Julian Schulz, Julian Schmitt, Martin Weitz, Georg von Freymann and Frank Vewinger: Dimensional crossover in a quantum gas of light; Nature Physics; DOI: 10.1038/s41567-024-02641-7; URL: www.nature.com/articles/s41567-024-02641-7