Researchers create extremely compressible gas from light particles


March 25, 2022 4:36 p.m IS

Bonn [Germany]March 25 (ANI): Researchers at the University of Bonn have created an extremely compressible gas from light particles.
The study was published in the journal Science.
The results of the study have confirmed the predictions of central theories in quantum physics. The findings could also point the way to innovative sensors that can measure the smallest forces.
If you plug the outlet of an air pump with your finger, you can still push its piston down. The reason for this is that gases are fairly easy to compress – unlike liquids, for example. If the pump contained water instead of air, it would be virtually impossible to move the piston even with the greatest effort.
Gases mostly consist of atoms or molecules that whirl through space at varying speeds. It’s quite similar to light. Its smallest building blocks are photons, which behave in some ways like particles, and these photons can also be treated as a gas, but one that behaves somewhat unusually. You can compress it with almost no effort under certain conditions. At least that’s what theory predicted.
Researchers have now been able to demonstrate precisely this effect in experiments for the first time. “To do this, we stored light particles in a tiny box made of mirrors,” explains Dr. Julian Schmitt from the IAP, Principal Investigator in the group of Prof. Dr. Martin Weitz.
Weitz said, “The more photons we put in, the denser the photon gas became.”
In general, the denser a gas is, the more difficult it is to compress. As with the clogged air pump, the piston can initially be pushed down very easily, but at some point it can hardly be moved any further, even with a lot of force. The Bonn experiments were initially similar: the more photons they put into the mirror box, the more difficult it became to compress the gas.
However, after a certain point, the behavior changed abruptly. As soon as the photon gas exceeded a certain density, it could suddenly be compressed with almost no resistance.

“This effect results from the rules of quantum mechanics,” explains Schmitt, who is also an associated member of the “Matter and Light for Quantum Computing” cluster of excellence and project leader in the Transregio Collaborative Research Center 185.
The reason is that the light particles have a “blur” – put simply, their position is a bit blurry. As they get very close at high densities, the photons begin to overlap. Physicists then speak of a “quantum degeneracy” of the gas. And it becomes much easier to compress such a quantum degenerate gas.
If the overlap is strong enough, the light particles merge into a kind of superphoton, a Bose-Einstein condensate. In very simplified terms, this process can be compared to the freezing of water: in the liquid state, the water molecules are disordered; the first ice crystals then form at the freezing point, which finally merge into an extensive, highly ordered layer of ice. “Islands of order” also form shortly before the formation of the Bose-Einstein condensate and become larger and larger as more photons are added.
Only when these islands have grown so much that the order extends over the entire mirror box with the photons does the condensate form. This is comparable to a lake on which individual ice floes have finally joined together to form a uniform surface. Of course, this requires a much larger number of light particles in an expanded box than in a small one.
“We were able to demonstrate this connection in our experiments,” emphasizes Schmitt.
To create a gas with variable numbers of particles and precisely defined temperatures, the researchers used a “heat bath”.
“We bring molecules into the mirror box that can absorb the photons,” says Schmitt.
“Then they emit new photons, which have the average temperature of the molecules – in our case almost 300 Kelvin, which is about room temperature,” he added.
The researchers also had to overcome another hurdle. Photon gases are usually not uniformly dense—there are many more particles in some places than others. This is due to the shape of the trap that usually contains them.
“We took a different approach in our experiments,” said Erik Bisley, first author of the publication.
“We capture the photons in a flat-bottomed mirror box that we created using a microstructuring process. This allowed us to create a homogeneous quantum gas from photons for the first time,” he concluded. (ANI)


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