In their experiments, physicists at the University of Bonn and the University of Kaiserslautern-Landau observed the dimensional crossover from one to two dimensions in a harmonically trapped gas of photons (light particles) and studied its properties. The photons were trapped in a dye microcavity where polymer nanostructures provided the trapping potential for the photon gas. By varying the aspect ratio of the trap, the researchers tuned from isotropic two-dimensional confinement to a highly elongated one-dimensional trapping potential. The team’s paper appears in the journal Nature Physics.
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. Image credit: University of Bonn.
“To create gases out of photons, we need to concentrate lots of photons in a confined space and cool them simultaneously,” said University of Bonn’s Dr. Frank Vewinger.
In their experiments, Dr. Vewinger and his colleagues 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 ultimately the photon gas condensed.
The dimensionality of the gas can be influenced by modifying the surface of the reflective surfaces.
“We were able to apply a transparent polymer to the reflective surfaces to create microscopically small protrusions,” said Dr. Julian Schulz, a physicist at the University of Kaiserslautern-Landau.
“These protrusions allow us to trap the photons in one or two dimensions and condense them.”
“These polymers act like a type of gutter, but in this case for light,” said Dr. Kirankumar Karkihalli Umesh, a physicist at the University of Bonn.
“The narrower this gutter is, the more one-dimensionally the gas behaves.”
In two dimensions, there is a precise temperature limit at which condensation occurs — similar to how water freezes at precisely 0 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,” Dr. Vewinger said.
“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 were to turn into a form of icy water at low temperatures without ever completely freezing when cooling down.
“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,” Dr. Vewinger said.
The authors 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.
This is still considered basic research at the moment but it is possible that it could open up new areas of application for quantum optical effects.
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K. Karkihalli Umesh et al. Dimensional crossover in a quantum gas of light. Nat. Phys, published online September 6, 2024; doi: 10.1038/s41567-024-02641-7
