Quantum Gas Made From Light Itself

in GEMSlast year

A new design of an optical microresonator brought us the first homogenous photon gas in which the photons act in a way atoms and molecules act in a normal gas. And thanks to quantum degeneration it becomes very easily compressible after reaching a certain density.


Photo by FLY:D on Unsplash

Gases are quite well compressible in general. And there is a simple rule associated with compressing gases. The denser the gas the harder it is to compress it. But it seems this rule isn’t absolute. Physicists from the Universität Bonn in Germany have created a gas out of photons. You know, out of light! In this case, the individual photons behave similarly to atoms and molecules in normal gases.

The leader of the research team Julian Schmitt and his colleagues built an optical microresonator. It was made by an area bound by mirrors into which they pumped photons. At first, the “light gas” behaved according to the general rule, and as the photon density grew it was harder and harder to compress. But when this “gas” reached a certain density, things changed. The “light gas” suddenly became completely compressible without any obvious resistance.

As Schmitt explains, this seeming magic behavior comes out of the reality of quantum mechanics. The thing is photons a somewhat unfocused when it comes to their existence in space. When they start getting too close at higher densities their positions in space start to overlap. This creates a quantum-degenerate gas that is more than easily compressible. The photons essentially become a single “superphoton” which is a certain phase of the Bose-Einstein condensate.

We have seen similar “light gases” before. But usually, they aren’t homogenous and have a different density at different places. This tends to be caused by the shape of the device in which such gas is created. As the original author of the study – Erik Busley – says, they used a microresonator with a microstructured flat bottom to create the gas and this allowed them to be the first to create a homogenous light gas.

These results will certainly be impactful for fundamental research. But they also promise interesting practical uses such as the creation of new types of sensors to measure minuscule forces.