By Zeeya Merali
The fuzzy dividing line between light and atoms has been blurred even further. Quantum physicists have created the first Bose-Einstein condensate using photons--a feat until now suspected to be possible only for atoms. The technique could be used to increase the efficiency of solar cells and lasers.
Bose-Einstein condensates (BECs) are a bizarre quantum phase of matter. They were first proposed in the 1920s by Satyendra Nath Bose and Albert Einstein, who reasoned that if certain atoms are chilled to within a fraction of absolute zero, quantum effects should take over. As a result, all the atoms are squeezed into the same quantum state, so that they "march in step," behaving collectively as though they are a sort of super-atom, explains quantum physicist Martin Weitz at the University of Bonn in Germany.
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In 1995, two experimental groups independently produced the first examples of BECs with rubidium and sodium atoms. In theory, physicists knew that it should also be possible to form a BEC using particles of light, or photons. But in practice it seemed near impossible because, unlike atoms, the number of photons in an experiment is not conserved. That means that when you try to chill photons they vanish from the experiment, becoming absorbed by surrounding atoms in the apparatus, says Weitz. "If you try to cool down a light bulb, it goes off--the light just disappears--and that's the big problem," he explains.
Light trap
Now Weitz and his colleagues have found a way to get light to stick around long enough for a BEC of photons to be created--details of the technique are published in the November 24 issue of Nature. To prevent the usually massless photons from escaping, the team trapped them in a cavity between two curved mirrors. The mirrors restricted the way the photons could move and vibrate--forcing them to behave as though they were atoms with a mass about ten billion times smaller than a rubidium atom.
To build a standard BEC, atoms must usually collide with each other, to even out their temperature. But photons, even those with a slight 'mass', interact too weakly to do this. So the team added dye molecules to the cavity; these absorbed and re-emitted the photons, helping them to reach thermal equilibrium. "The magic of BEC formation happens when you pump more and more photons into the cavity until suddenly, no more can enter this thermal equilibrium, so they condense out," says Weitz. These extra photons undergo a quantum transition, dropping into the same low energy state and forming a BEC.
The team could tell when the transition had occurred because the small number of photons in the BEC formed an intense beam of yellow light -- like a laser -- in the centre of the cavity, surrounded by the "gas" of remaining normal photons. To double-check that they were seeing a BEC of light, the researchers repeated the experiment with different numbers of photons. In each case, once the transition had taken place, they measured the spectrum of light leaking from the cavity and found that it matched theoretical predictions for the corresponding BEC.
Fundamental difference
Wolfgang Ketterle at the Massachusetts Institute of Technology in Cambridge--who won a share of the 2001 Nobel Prize in Physics for leading one of the groups that first generated a BEC with atoms--describes the work as "a spectacular piece of physics" that removes one more distinction between atoms and light. "When I give lectures about Bose-Einstein condensates, I often talk about why they can't be made using photons, to highlight this one fundamental difference between photons and atoms--but now even that difference has gone," he says.
Matthias Weidemüller, a quantum physicist at the University of Freiberg in Germany, says that the idea behind the experiment is "truly ingenious" whereas, ironically, carrying it out is relatively easy. "Compared to Bose-Einstein condensation with ultracold atoms, the current experiment is ridiculously simple," he says.
The technique could one day have practical applications for collecting and focusing sunlight, says Weidemüller. Whereas an ordinary lens can concentrate sunlight in solar cells on a clear day, the BEC technique has the advantage that it could also collect light scattered in all directions on a cloudy day, he explains.
Photon BECs could also provide an alternative way of generating laser beams, says Ketterle. "It is too early to say how competitive possible applications could be, but they should be explored," he adds.
