Physicists create extremely compressible 'light gas'

If you stick your finger in the outlet of the air pump, you can still push its piston down. The reason: Gases are fairly compressible - like liquids, for example. If the pump contained water instead of air, it would be essentially impossible to move the piston, even with the greatest effort.

Gases are usually composed of atoms or molecules that swirl more or less rapidly through space. It is quite similar to light: Its smallest building blocks are photons, which in some ways behave like particles. And these photons can also be thought of as a gas, but one photon behaves a little differently: You can compress it under certain conditions with almost no effort. At least that's what the theory predicts.

Photons in the mirror box

Researchers from the Institute of Applied Physics (IAP) at the University of Bonn demonstrated this effect in experiments.

Dr Julian Schmitt of IAP- Principal Investigator in Professor Dr Martin Weitz's team, explains: 'To do this, we stored the light particles in a small box made of a mirror. The more photons we put in there, the denser the gas photons become."

The general rule is: The denser the gas, the harder it is to compress. This is also the case with plugged-in air pumps - the piston can be pushed down very easily at first, but at some point it can hardly move anymore, even with a lot of force applied. The early Bonn experiments were similar: The more photons they placed in the mirror box, the more difficult it became to compress the gas.

However, the behavior changes abruptly at a certain time: As soon as the photon gas exceeds a particular density, it can suddenly be compressed with almost no resistance. "This effect is the result of the rules of quantum mechanics," explains Schmitt, who is also an associate member of the "Matter and Light for Quantum Computing" Excellence Cluster and project leader. at Transregio Collaborative Research Center 185. Reason: Light particles exhibit "opacity" - put simply, their positions are slightly blurred. When they get very close together at high densities, the photons start to overlap. The physicists then also talked about the "quantum degeneracy" of the gas. And it becomes much easier to compress such a quantum degenerate gas.

Self-organizing photons

If the overlap is strong enough, the light particles merge to form a type of superphoton, the Bose-Einstein condensate. Simply put, the process can be compared to the freezing of water: In the liquid state, the water molecules are disordered; then, at the freezing point, ice crystals first form, eventually merging into a highly ordered, expanded layer of ice. "Order islands" are also formed just before the Bose-Einstein condensate, and they grow larger as more photons are added.

Condensate is only formed when these islands grow so much that the order extends across the entire mirror box containing the photons. This can be compared to a lake on which independent ice sheets eventually coalesce to form a uniform surface. Naturally, this requires a much larger number of light particles in an expanded box than in a small one. "We were able to demonstrate this relationship in our experiments," Schmitt points out.

To create a gas with a variable number of particles and a clearly defined temperature, the researchers use a "heat bath": "We put the molecules in a mirror box that can absorb the photons," Schmitt explained. "They then emit new photons whose average temperature is the same as that of the molecules - in our case, just under 300 Kelvin, which is room temperature."

The researchers also had to overcome another obstacle: Photon gasses are often not uniformly dense – there are far more particles in some places than in others. This is due to the shape of the traps they usually contain. "We took a different approach in our experiments," said Erik Busley, first author of the publication. "We captured the photons in a flat-bottomed mirror box, which we created using a microstructure method. This allowed us to create a homogeneous quantum gas of photons for the first time."

In the future, the quantum-enhanced compressibility of gases will allow research into new sensors that can measure extremely small forces. Besides the technological promise, the results are also of great interest to basic research.