Scientists have turned photons, the wave/particles we see as light, into one huge super-particle. The photons share the same energy level and can’t be distinguished from each other.
Bose–Einstein condensation (BEC)—the macroscopic ground-state accumulation of particles with integer spin (bosons) at low temperature and high density—has been observed in several physical systems including cold atomic gases and solid-state quasiparticles. However, the most omnipresent Bose gas, blackbody radiation (radiation in thermal equilibrium with the cavity walls) does not show this phase transition. In such systems photons have a vanishing chemical potential, meaning that their number is not conserved when the temperature of the photon gas is varied; at low temperatures, photons disappear in the cavity walls instead of occupying the cavity ground state. Theoretical works have considered thermalization processes that conserve photon number (a prerequisite for BEC), involving Compton scattering with a gas of thermal electrons or photon–photon scattering in a nonlinear resonator configuration. Number-conserving thermalization was experimentally observed for a two-dimensional photon gas in a dye-filled optical microcavity, which acts as a ‘white-wall’ box. Here we report the observation of a Bose–Einstein condensate of photons in this system. The cavity mirrors provide both a confining potential and a non-vanishing effective photon mass, making the system formally equivalent to a two-dimensional gas of trapped, massive bosons. The photons thermalize to the temperature of the dye solution (room temperature) by multiple scattering with the dye molecules. Upon increasing the photon density, we observe the following BEC signatures: the photon energies have a Bose–Einstein distribution with a massively populated ground-state mode on top of a broad thermal wing; the phase transition occurs at the expected photon density and exhibits the predicted dependence on cavity geometry; and the ground-state mode emerges even for a spatially displaced pump spot. The prospects of the observed effects include studies of extremely weakly interacting low-dimensional Bose gases and new coherent ultraviolet sources.
First Bose–Einstein condensate of photons could help build solar cells and lasers.
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.
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 Nature today. 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.
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.
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 “super-photon.”
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 short-wave light – i.e. in the UV or X-ray range,” explained Jan 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 team trapped its photons between two concave mirrors that are separated by a maximum of 1.5 µm. This distance (to within an integer number) defines the maximum wavelength – or minimum energy – of a photon that is confined longitudinally within the cavity between the mirrors. The cavity is filled with a dye that is held at room temperature – and, crucially, the thermal energy of the dye is about 1% of the photon energy.
This large energy difference means that it is highly unlikely that additional photons will emerge from the dye, or that the dye will completely absorb a photon. Instead, the photons collide with the dye molecules, giving up or receiving small amounts of energy. These interactions cool the photons to room temperature – which is cold enough to create a photon BEC – while preserving the number of photons.
The team created the BEC by firing a laser into the cavity to fill it with photons. The laser was then kept on throughout the experiment to make up for photons that were lost at the mirrors and imperfections in the cavity. Some of the photons pass through one of the mirrors to a spectrometer, which measures the distribution of photon energies in the cavity. At low laser intensities the cavity contains a broad range of photon energies with a sharp cut-off at the cavity’s minimum energy.
Critical number of photons
When the laser intensity is increased, the number of photons in the cavity rises and the broad distribution endures until the photon number reaches about 60,000. Above this critical value, according to Weitz, the photon gas is dense enough for a BEC to form – much like a liquid drop condensing in a gas.
The team knows that the BEC has formed because a large peak in the photon energy spectrum emerges just above the cut-off energy. This peak corresponds to a large number of photons piling into the lowest energy state of the cavity. As the laser intensity is increased further, the number of photons in the BEC reaches millions.
To convince themselves that the peak is related to a BEC, rather than the cavity behaving like a laser, the researchers repeated the experiment at several different separation distances. They found that the peak always emerged at the same photon density – something that would not be seen in a laser, according to Weitz.
The cavity has a planar design, which means that the photons are confined to two dimensions. As a result of the longitudinal confinement, they behave as if they are particles with an “effective mass” corresponding to the cut-off energy. This mass is still extremely small, which is why photons will form a BEC at room temperature and don’t need to be cooled to micro-Kelvin temperatures like atoms.
Interactions between the photons are much weaker than those between atoms and this means that photons can form a true 2D BEC. Atoms, on the other hand can only form a 3D BEC.
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