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Simultaneous Cooling, Concentrating of Light Creates Superphoton
Nov 2010
BONN, Germany, Nov. 24, 2010 — By devising a way to cool and control light at the same time, a "gas" of photons was made to form the ultracold, high-density quantum state known as a Bose-Einstein condensate. Such photon-filled condensates, thought to be impossible to create until recently, could potentially serve as a new source of light for lasers that work in the x-ray range or in more powerful computer chips.

An illustration of the superphoton. (Image: Jan Klaers, University of Bonn)

Bose–Einstein condensation — the macroscopic accumulation of particles in a single, lowest-energy quantum state — has now been observed in several systems comprising identical particles with integer spin (known as bosons). Although photons are bosons, a simple system of photons in thermal equilibrium with an optical cavity does not condense, because as the photons cool they can be absorbed by the cavity walls and disappear. For Bose–Einstein condensation to occur, it was necessary to find a way to cool light while concentrating it at the same time.

Dr. Martin Weitz and his colleagues Jan Klärs, Julian Schmitt and Dr. Frank Vewinger at the University of Bonn achieved this by confining photons in a dye-filled mirrored microcavity small enough to put a lower limit on each photon’s energy. This, and the cavity’s curved mirrors, make the photons behave as if they were particles with mass, trapped in two dimensions. Repeated interaction with the dye molecules brought the photons to room temperature which, in this confined system, is cold enough to allow Bose–Einstein condensation, where the Rubidium atoms behave like a single huge "superparticle."

How warm is light?

When the tungsten filament of a light bulb is heated, it starts glowing — first red, then yellow, and finally bluish. Thus, each color of the light can be assigned a "formation temperature." Blue light is warmer than red light, but tungsten glows differently than iron, for example. This is why physicists calibrate color temperature based on a theoretical model object, a so-called black body. If this body were heated to a temperature of 5500 centigrade, it would have about the same color as sunlight at noon. In other words: noon light has a temperature of 5500 degrees Celsius or not quite 5800 Kelvin (the Kelvin scale does not know any negative values; instead, it starts at absolute zero or -273 centigrade; consequently, Kelvin values are always 273 degrees higher than the corresponding Celsius values).

When a black body is cooled down, it will at some point radiate no longer in the visible range; instead, it will only give off invisible infrared photons. At the same time, its radiation intensity will decrease. The number of photons becomes smaller as the temperature falls. This is what makes it so difficult to get the quantity of cool photons that is required for Bose-Einstein condensation to occur.

And yet, the Bonn researchers succeeded by using two highly reflective mirrors between which they kept bouncing a light beam back and forth. Between the reflective surfaces there were dissolved pigment molecules with which the photons collided periodically. In these collisions, the molecules "swallowed" the photons and then "spit" them out again.

The creators of the superphoton are Julian Schmitt (left), Jan Klaers, Dr. Frank Vewinger and professor Dr. Martin Weitz (right). (Image: Volker Lannert / University of Bonn)

"During this process, the photons assumed the temperature of the fluid," explained Weitz. "They cooled each other off to room temperature this way, and they did it without getting lost in the process."

A condensate made of light

The Bonn physicists then increased the quantity of photons between the mirrors by exciting the pigment solution using a laser. This allowed them to concentrate the cooled-off light particles so strongly that they condensed into a "superphoton."

This photonic Bose-Einstein condensate is a completely new source of light that has characteristics resembling lasers. But compared to lasers, they have a decisive advantage, "We are currently not capable of producing lasers that generate very shortwave light – i.e. in the UV or x-ray range," said Klärs. "With a photonic Bose-Einstein condensate this should, however, be possible."

This prospect should primarily please chip designers. They use laser light for etching logic circuits into their semiconductor materials. How fine these structures can be is limited by the wavelength of the light, among other factors. Long-wavelength lasers are less well suited to precision work than short-wavelength ones — it is as if you tried to sign a letter with a paintbrush.

X-ray radiation has a much shorter wavelength than visible light. In principle, x-ray lasers should thus allow applying much more complex circuits on the same silicon surface. This would allow creating a new generation of high-performance chips — and consequently, more powerful computers for end users. The process could also be useful in other applications such as spectroscopy or photovoltaics.

The scientists report their discovery in the Nov. 25 issue of the journal Nature.

For more information, visit: 

bose-einstein condensate
A group of atoms that have been cooled to the point that they have minimum motion and share the same, lowest possible quantum state.
color temperature
A colorimetric concept related to the apparent visual color of a source (not its temperature). For a blackbody, the color temperature is equal to the temperature in kelvin.
Basic ScienceBose-Einstein condensatebosonsColor TemperatureDr. Frank VewingerDr. Martin Weitzdye-filled mirrored microcavityEuropeGermanyinfrared photonsJan KlärsJulian Schmittlaserslight sourcesmirrorsoptical cavityopticsphoton-filled condensatephotonsphotovoltaicsResearch & Technologyrubidium atomsspectroscopysuperparticlesuperphotonthermal equilibriumtungsten filamentultracold high-density quantum stateUniversity of BonnUVx-rays

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