• Physicists create extremely compressible

    From ScienceDaily@1:317/3 to All on Thursday, March 24, 2022 22:30:44
    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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