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U of Bonn physicists create compressible optical quantum gas that could enable new types of sensors

By Sandra Helsel posted 28 Mar 2022

(Phys.org) Inside Quantum Technology summarizes research findings at the University of Bonn where physicists have created a gas of light particles that can be extremely compressed. Their results confirm the predictions of central theories of quantum physics. The findings could also point the way to new types of sensors that can measure minute forces.
Gases usually consist of atoms or molecules that swirl more or less quickly through space. It is quite similar with light: Its smallest building blocks are photons, which in some respect behave like particles. And these photons can also be treated as a gas, however, one that behaves somewhat unusually: Theory predicts you can compress it under certain conditions with almost no effort.
Researchers from the Institute of Applied Physics (IAP) at the University of Bonn have now demonstrated this very 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 of the IAP, who is a principal investigator in the group of Prof. Dr. Martin Weitz. “The more photons we put in there, the denser the photon gas became.”
To create a gas with variable particle number and well-defined temperature, the researchers use a “heat bath”: “We insert molecules into the mirror box that can absorb the photons,” Schmitt explains. “Subsequently, they emit new photons that on average possess the temperature of the molecules—in our case, just under 300 Kelvin, which is about room temperature.”
The researchers also had to overcome another obstacle: Photon gases are usually not uniformly dense—there are far more particles in some places than in others. This is due to the shape of the trap which they are usually contained in. “We took a different approach in our experiments,” says Erik Busley, first author of the publication. “We capture the photons in a flat-bottom mirror box that we created using a microstructuring method. This enabled us to create a homogeneous quantum gas of photons for the first time.”
In the future, the quantum-enhanced compressibility of the gas will enable research into novel sensors that could measure tiny forces. Besides technological prospects, the results are also of great interest for fundamental research.

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