Physicists create extremely compressible 'gas of light'
Paving the way to new types of highly sensitive sensors
Date:
March 24, 2022
Source:
University of Bonn
Summary:
Researchers 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.
FULL STORY ========================================================================== Researchers at the University of Bonn 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. The study is published in the journal Science.
==========================================================================
If you plug the outlet of an air pump with your finger, you can still
push its piston down. The reason: Gases are fairly easy to compress --
unlike 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 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: You can compress it under certain conditions with almost no effort. At least that is what theory predicts.
Photons in the mirror box 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." The rule is usually: The denser a gas,
the harder it is to compress. This is also the case with the plugged air
pump -- at first the piston can be pushed down very easily, but at some
point it can hardly be moved any further, even when applying 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, the behavior changed abruptly at a certain point: As soon
as the photon gas exceeded a specific 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 associate member
of the Cluster of Excellence "Matter and Light for Quantum Computing" and project leader in the Transregio Collaborative Research Center 185. The
reason: The light particles exhibit a "fuzziness" -- in simple terms,
their location is somewhat blurred.
As they come very close to each other at high densities, the photons begin
to overlap. Physicists then also speak of a "quantum degeneracy" of the
gas. And it becomes much easier to compress such a quantum degenerate gas.
Self-organized photons If the overlap is strong enough, the light
particles fuse to form a kind of super-photon, a Bose-Einstein
condensate. In very simplified terms, this process can be compared
to the freezing of water: In a liquid state, the water molecules are disordered; then, at the freezing point, the first ice crystals form,
which eventually merge into an extended, highly ordered ice layer.
"Islands of order" are also formed just before the formation of the Bose- Einstein condensate, and they become larger and larger with the further addition of photons.
The condensate is formed only when these islands have grown so much that
the order extends over the entire mirror box containing the photons. This
can be compared to a lake on which independent ice floes have finally
joined together to form a uniform surface. Naturally, this requires a
much larger number of light particles in an extended box as compared to a
small one. "We were able to demonstrate this relation in our experiments," Schmitt points out.
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.
Funding: The study was supported by the German Research Foundation (DFG)
within the collaborative research center TRR 185 "OSCAR -- Open System
Control of Atomic and Photonic Matter" and the cluster of excellence
"Matter and Light for Quantum Computing (ML4Q)," and by the European
Union within the framework of the quantum flagship project "PhoQuS --
Photons for Quantum Simulation." Video:
https://youtu.be/lyrd5srcyEo
========================================================================== Story Source: Materials provided by University_of_Bonn. Note: Content
may be edited for style and length.
========================================================================== Journal Reference:
1. Erik Busley, Leon Espert Miranda, Andreas Redmann, Christian
Kurtscheid,
Kirankumar Karkihalli Umesh, Frank Vewinger, Martin Weitz and Julian
Schmitt. Compressibility and the Equation of State of an Optical
Quantum Gas in a Box. Science, 2022 DOI: 10.1126/science.abm2543 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/03/220324143745.htm
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