Scientists created a one-dimensional gas out of light

In a joint experiment by the University of Bonn and the University of Kaiserslautern-Landau, physicists have created a one-dimensional gas out of light. This allowed scientists to test theoretical predictions about the transition into this exotic state of matter for the first time.

Imagine standing by a swimming pool and deciding to fill it with more water. You use a garden hose to shoot a water jet into the pool. The water level goes up a little where the jet hits but quickly spreads out, so the overall effect on the pool’s water level is minimal.

The situation is different if you use the same hose to fill a gutter. The jet creates a noticeable wave in the gutter because the walls of the gutter keep the water from spreading out. The narrower the gutter, the higher the wave becomes, making the effect more pronounced.

In this study, physicists explored whether gases made of light particles (photons) can exhibit similar effects of dimensionality to water.

Dr. Frank Vewinger from the IAP, who is also a member of the transdisciplinary research area, said, “To create these types of gases, we need to concentrate lots of photons in a confined space and cool them simultaneously.”

Scientists filled a tiny container with a dye solution and then excited it with a laser. The photons bounced back and forth between the container’s reflective walls.

Photons collided with dye molecules and were cooled until the photon gas condensed. The dimensionality of the gas can be adjusted by changing the surface of the reflective walls. Scientists used a high-resolution structuring method to modify the reflective surfaces of the photon container for their experiment.

Julian Schulz from the RPTU said, “We were able to apply a transparent polymer to the reflective surfaces to create microscopically small protrusions. These protrusions allow us to trap the photons in one or two dimensions and condense them.”

Kirankumar Karkihalli Umesh, the study’s lead author, said, “These polymers act like a type of gutter, but in this case for light. The narrower this gutter is, the more one-dimensionally the gas behaves.”

It is different when we create a one-dimensional gas instead of a two-dimensional one.

Swinger said, “So-called thermal fluctuations occur 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.”

In one-dimensional systems, fluctuations disrupt the uniformity, making different gas parts behave differently. This causes the phase transition, clear-cut in two dimensions, to become more “smeared out” in one dimension. Despite this, the system’s behavior is still governed by quantum physics, like in two-dimensional gases, known as degenerate quantum gases. It’s similar to water turning into a slushy, icy state rather than freezing completely when cooled 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,” explains Vewinger.

Scientists have now demonstrated that one-dimensional photon gases have no precise condensation point. By making minor adjustments to the polymer structures, researchers can now study phenomena that occur at the transition between different dimensionalities in detail. While this is currently basic research, it could potentially lead to new applications for quantum optical effects.

Journal Reference:

1. 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