Scientists create one-dimensional gas made of light

Physicists from the University of Bonn and the University of Kaiserslautern-Landau (RPTU) have successfully created a one-dimensional gas using light, allowing them to experimentally test theoretical predictions about this exotic state of matter for the first time.

Their innovative method has the potential to advance the study of quantum effects.

Imagine standing by a swimming pool and wanting to increase the water level. By using a garden hose to produce a high-arching jet of water that falls onto the pool’s surface, you observe a minimal increase in the water level as the falling water quickly disperses. However, if you were to direct the jet into a gutter, the water would create a wave due to the confinement provided by the gutter’s walls.

The physicists from the Institute of Applied Physics (IAP) at the University of Bonn, in collaboration with colleagues at RPTU, have explored whether similar effects of dimensionality can be achieved with gases made out of light particles.

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

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. Artistic illustration: IAP/Uni Bonn

In their experiment, researchers carefully prepared a minute container filled with a potent dye solution, which they then stimulated using a precise laser. The resulting photons ricocheted between the reflective walls of the container, undergoing a remarkable cooling process whenever they interacted with a dye molecule, ultimately leading to the condensation of the photon gas.

The dimensional properties of the gas were found to be modifiable by altering the surface of the reflective walls. This pioneering study was a result of collaboration between the researchers at the IAP and the esteemed research group led by Prof. Dr. Georg von Freymann from the RPTU. Leveraging a high-resolution structuring method, they successfully applied it to the reflective surfaces of the photon container, marking a significant advancement in this field of study.

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

Kirankumar Karkihalli Umesh witht the centerpiece of the experiment – a resonator mirror printed with polymer structures. Credit: Volker Lannert/ Uni Bonn

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

In two dimensions, a specific temperature threshold triggers condensation, much like water freezing at exactly zero degrees Celsius. Physicists refer to this as 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 disrupt the uniformity of one-dimensional systems, causing different regions within the gas to behave differently. Consequently, the phase transition, while still precisely defined in two dimensions, becomes increasingly “smeared out” in more one-dimensional systems. Nevertheless, its properties are still governed by quantum physics, similar to the case of two-dimensional gases, which are known as degenerate quantum gases. It’s akin to water transforming into a slushy state at low temperatures without completely freezing.

“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.

The research teams successfully proved that one-dimensional photon gases lack a precise condensation point. With minor adjustments to polymer structures, a detailed exploration of phenomena at the transition between dimensionalities is now feasible. Although currently classified as basic research, this breakthrough has the potential to unveil 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, 2024 DOI: 10.1038/s41567-024-02641-7

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