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November 25, 2010

Scientists in Germany have disproven assumptions that creating a "super-photon" was impossible, by finding a solution with a comparatively simple experiment.

A photo illustration of the photonic Bose-Einstein condensate
By cooling and trapping light, scientists made a super-photonImage: Universität Bonn

A team of physicists, lead by the University of Bonn's Martin Weitz, have managed to create a Bose-Einstein condensate out of photons, or particles of light. This discovery could be used to develop better microchips for computers or more efficient solar energy technology, according to their research which was published in the journal Nature on Thursday.

Bose-Einstein condensates (BEC) were first proposed in the 1920s by Satyendra Nath Bose and Albert Einstein, who asserted that atoms, chilled to very near absolute zero, would take on quantum characteristics, so that the atoms "overlap, and behave as a giant meta-wave," or a sort of super-atom, wherein a collection of atoms all act in concert, Weitz told Deutsche Welle.

Not everyone in the field of quantum optics is happy with the term super-atom or super-photon though, such as Aephraim Steinberg, a quantum physicist at the University of Toronto.

"I find the word a little confusing and misleading, so I wouldn’t really want to call this a super photon," Steinberg said.

Within the BEC there is still a collection of weakly interacting particles; they don’t merge together to create a new molecule or new solid, but they take on an additional property that they all want to behave the same way.

Prof. Dr. Martin Weitz stands near the experiment in the lab at the University of Bonn
Martin Weitz and his team worked on the experiment for three yearsImage: DW/Stuart Tiffen

"So it's no more a super-atom than an army is a super person; there may be ten thousand people marching in the same direction, but they're not some sort of new creature," Steinberg said.

The BEC state was previously believed to apply only to atoms and not to photons, since the cooling process would force the photons to be absorbed by surrounding atoms in the apparatus.

"If you cool down light, normally the light goes away," Weitz explained. "If you turn off a light bulb then the filament cools down and it changes its color from yellow to red, as the number of photons is reduced, becoming dimmer and dimmer until they disappear entirely."

Photons acting like atoms

But now Weitz, along with Jan Klaers, Julian Schmitt and Frank Vewinger, has managed to create a photonic BEC at room temperature by trapping photons between two concave mirrors.

The experiment also made use of liquid dye pigment that cooled the photons down to room temperature while they were trapped between the two mirrors. When enough photons were pumped into this apparatus, a BEC state was observed "which appeared as a yellow peak in the middle and there the photons marched in step," Weitz said.

The discovery took three years of research and is being hailed as a major breakthrough in the field of quantum physics. Matthias Weidemueller, a quantum physicist at the University of Freiburg, said in the journal Nature that the experiment was "truly ingenious."

"Compared to Bose-Einstein condensation with ultracold atoms, the current experiment is ridiculously simple," he said.

But some in the field maintain that, while the results are welcome and that the concept of a BEC of photons was controversial, it's not completely unexpected. Aephraim Steinberg, a quantum physicist at the University of Toronto, says quantum mechanics basically dictate that all bosons, a category which includes Rubidium-87 atoms used in atomic BEC, as well as photons, want to act in the same way.

The equipment at the lab at the University of Bonn
One potential application would be the creation of shortwave lasersImage: DW/Stuart Tiffen

"I think there's been a lot of confusion and I think (this experiment) helps clarify some fundamental physics to see that, at a deep level, all particles are really the same, whether it’s a boson of light or a boson of rubidium, they really do have the same properties if you put them in the right circumstances," Steinberg said in a telephone interview.

Putting it to work

Weitz has a couple of ideas for applications resulting from the research at the University of Bonn. Creating shortwave lasers in the X-ray or ultra-violet spectrum is one such idea.

Such shortwave lasers would be well-suited to the manufacture of computer chips as the process uses lasers to etch logic circuits onto semiconductor materials. The precision of shortwave lasers is superior to those with longer wavelengths, which would allow chip designers to create more complex circuitry on the same area of silcon.

Another possible application using this apparatus, but with less light, Weitz said, would be to concentrate the light for solar cells. By placing the apparatus before a solar cell, "you could save a lot of money because you could use smaller solar cells but nevertheless, get all the light," Weitz said.

Weitz and his colleagues will now be focusing on learning more about the photonic BEC they've created, and also attempting to simplify the apparatus by switching out the liquid dye currently used, for a solid-state dye, which would require less equipment to manage.

Author: Stuart Tiffen
Editor: Nicole Goebel