Researchers from the University of Bonn and the University of Kaiserslautern-Landau (RPTU) have successfully developed a one-dimensional gas composed of light. This groundbreaking achievement allows them to validate theoretical expectations regarding the transition into this unique form of matter for the first time. The innovative technique utilized in their experiment holds potential for exploring quantum phenomena. The findings are documented in the journal “Nature Physics.”
Envision yourself by a swimming pool, contemplating how to add more water. You take a garden hose and direct a stream of water in a high arc, causing it to splash into the pool. The water level rises slightly at the splash point, but this increase is minimal because the water quickly spreads out across the pool’s surface.
In contrast, if you aim your jet of water into a gutter, it creates a visible wave where the water strikes, as the gutter walls prevent the water from spreading out freely. Instead, it can only move along the gutter’s length. The narrower the gutter, the taller the resulting wave, which gives it a “more one-dimensional” characteristic.
Physicists from the Institute of Applied Physics (IAP) at the University of Bonn, in collaboration with the RPTU, investigated whether similar one-dimensional effects could exist in gas formed from light particles. “To create these gas types, we need to concentrate a large number of photons in a confined area while also cooling them,” states Dr. Frank Vewinger from the IAP, who is involved in the University of Bonn’s transdisciplinary research area titled “Matter.”
Microscopically Small Gutters
In their experiment, the scientists filled a miniature container with a dye solution and illuminated it with a laser. The resulting photons ricocheted between the container’s reflective walls. Each time a photon struck a dye molecule, it was cooled until the photon gas ultimately condensed.
The dimensionality of the gas can be altered by changing the surface structure of the reflective walls. The researchers from IAP worked with a team led by Prof. Dr. Georg von Freymann from RPTU on this project. They adapted a high-resolution structuring method for use on the reflective surfaces of the photon container. “We applied a transparent polymer to the reflective surfaces to create tiny protrusions,” explains Julian Schulz from RPTU. “These protrusions help us to trap the photons in one or two dimensions and condense them.”
“These polymers function like a type of gutter, but specifically for light,” remarks Kirankumar Karkihalli Umesh, the primary author of the study. “The narrower the gutter, the more one-dimensionally the gas behaves.”
Thermal Fluctuations Disrupt the Condensation Point
In a two-dimensional setup, there’s a specific temperature threshold where condensation occurs—much like how water freezes exactly at zero degrees Celsius. Physicists refer to this as a phase transition. “However, the scenario differs when we create a one-dimensional gas instead of a two-dimensional one,” Vewinger explains. “In photon gases, thermal fluctuations happen, but they are minimal in two dimensions, having little effect. In one dimension, however, these fluctuations can create significant disturbances.”
These fluctuations disrupt the order within one-dimensional systems, causing different areas of the gas to behave inconsistently. Consequently, the distinct phase transition seen in two dimensions becomes increasingly “smeared out” in one-dimensional systems. Nevertheless, quantum physics still governs its properties, similar to two-dimensional gases, which are known as degenerate quantum gases. It’s akin to water transitioning into a slushy form at low temperatures without completely freezing. “We have now investigated this behavior during the transition from a two-dimensional to a one-dimensional photon gas for the first time,” Vewinger elaborates.
The research teams discovered that one-dimensional photon gases lack a precise condensation point. With slight adjustments to the polymer structures, they can explore phenomena occurring at the transition between different dimensions in greater detail. Although this is still classified as fundamental research, it has the potential to pave the way for new applications in quantum optical effects.
Participating Institutes and Funding:
The following institutions contributed to the study: the IAP at the University of Bonn, the Fraunhofer Institute for Industrial Mathematics (ITWM) in Kaiserslautern, and the University of Kaiserslautern-Landau (RPTU). The research was supported by the European Research Council (ERC) of the European Union and the German Research Foundation (DFG) as part of Collaborative Research Centre TRR 185.
