Could photons, light particles, really condense? And how will this "liquid
light" behave? Condensed light is an example of a Bose-Einstein condensate:
The theory has been there for 100 years, but University of Twente
researchers have now demonstrated the effect even at room temperature. For
this, they created a micro-size mirror with channels in which photons
actually flow like a liquid. In these channels, the photons try to stay
together as group by choosing the path that leads to the lowest losses, and
thus, in a way, demonstrate "social behavior." The results are published in
Nature Communications.
A Bose-Einstein condensate (BEC) is typically a sort of wave in which the
separate particles can not be seen anymore: There is a wave of matter, a
superfluid that typically is formed at temperatures close to absolute zero.
Helium, for example, becomes a superfluid at those temperatures, with
remarkable properties. The phenomenon was predicted by Albert Einstein
almost 100 years ago, based on the work of Satyendra Nath Bose; this state
of matter was named for the researchers. One type of elementary particle
that can form a Bose-Einstein condensate is the photon, the light particle.
UT researcher Jan Klärs and his team developed a mirror structure with
channels. Light traveling through the channels behaves like a superfluid and
also moves in a preferred direction. Extremely low temperatures are not
required in this case, and it works at room temperature.
The structure is the well-known Mach-Zehnder interferometer, in which a
channel splits into two channels, and then rejoins again. In such
interferometers, the wave nature of photons manifests, in which a photon can
be in both channels at the same time. At the reunification point, there are
now two options: The light can either take a channel with a closed end, or a
channel with an open end. Jan Klärs and his team found that the liquid
decides for itself which path to take by adjusting its frequency of
oscillation. In this case, the photons try to stay together by choosing the
path that leads to the lowest losses—the channel with the closed end. You
could call it "social behavior," according to researcher Klärs. Other types
of bosons, like fermions, prefer staying separate.
The mirror structure somewhat resembles that of a laser, in which light is
reflected back and forth between two mirrors. The major difference is in the
extremely high reflection of the mirrors: 99.9985 percent. This value is so
high that photons don't get the chance to escape; they will be absorbed
again. It is in this stadium that the photon gas starts taking the same
temperature as room temperature via thermalization. Technically speaking, it
then resembles the radiation of a black body: Radiation is in equilibrium
with matter. This thermalization is the crucial difference between a normal
laser and a Bose-Einstein condensate of photons.
In superconductive devices at which the electrical resistance becomes zero,
Bose-Einstein condensates play a major role. The photonic microstructures
now presented could be used as basic units in a system that solves
mathematical problems like the Traveling Salesman problem. But primarily,
the paper shows insight into yet another remarkable property of light.
Reference:
Mario Vretenar et al, Modified Bose-Einstein condensation in an optical
quantum gas, Nature Communications (2021).
DOI: 10.1038/s41467-021-26087-0
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Physics