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[                    ]
A gas made from light becomes easier to compress as you squash it

Particles of light called photons can be trapped inside mirrors to
form a gas with unusual properties

      
Physics 24 March 2022

By Alex Wilkins

The photons are trapped inside two nanoscale mirrors

Volker Lannert/University of Bonn

A gas made of particles of light, or photons, becomes easier to
compress the more you squash it. This strange property could prove
useful in making highly sensitive sensors.

While gases are normally made from atoms or molecules, it is possible
to create a gas of photons by trapping them with lasers. But a gas
made this way doesn't have a uniform density - researchers say it
isn't homogeneous, or pure - making it difficult to study properly.

Now Julian Schmitt at the University of Bonn, Germany, and his
colleagues have made a homogeneous photon gas for the first time by
trapping photons between two nanoscale mirrors.

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They then moved one of the mirrors to measure the compressibility of
the photon gas and derive basic properties about it.

Read more: Photons could be 'split' in two to create a weird new form
of light

"We can consider the system to be like an air pump, but it's not
filled with air, it's filled with light," says Schmitt. "We compress
it and look at how it responds. In this way, we can learn about very
fundamental properties."

Surprisingly, the team found the gas could be compressed with no
resistance, becoming easier to compress with more force. A normal gas
becomes harder to compress the more you squeeze it because the
molecules, forced closer together, repel each other.

This doesn't happen with the homogeneous photon gas, thanks to
quantum mechanics, which allows the photons to act as both particles
and waves. "These photons behave as smeared-out waves that eventually
start to overlap, and then these particles become indistinguishable,"
says Schmitt.

As well as investigating fundamental states of matter, Schmitt says
this photon gas could be used to make highly sensitive sensors for
measuring forces. "[It] would allow you to have the tiniest forces
leading to an appreciable volume change that you could actually
measure," says Schmitt.

Journal reference: Science, DOI: 10.1126/science.abm2543

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