A Bose-Einstein Condensate is a strange substance. When a set of appropriate particles are cooled so that they all occupy a ground state, they begin to exhibit wave-like properties that enable them to behave as a superfluid, moving without any turbulence or resistance. But getting enough particles, such as atoms, to occupy a single low-energy state requires that they be chilled down to a fraction of a Kelvin above absolute zero, which meant that the first observation of a BEC didn't occur until 1995. Now, researchers have found a way to create a room-temperature BEC, but it required the use of unexpected material: light.

Light, which is constantly being absorbed, reflected, and re-emitted, isn't the easiest thing to pin down. Getting a collection of photons to enter a single energy state is even harder. But a team of researchers at Germany's Institut für Angewandte Physik figured out how to modify a well-known system—the laser—to force a large collection of photons into a sort of thermal equilibrium.

A simple laser is just a dye trapped between two mirrors. When the dye is pumped into a higher energy state, it will emit photons of a defined energy as it relaxes back to a ground state. As those photons bounce between the mirrors, they induce other pumped dye molecules to emit similar photons, gradually creating a large collection of trapped photons that are quite similar.

This system never reaches equilibrium, however, because there are constant losses; the mirrors aren't perfect, some light is harvested to create the laser beam, some dye molecules relax to other energy states or by colliding with solvent molecules, and so on. If the mirrors are far enough apart, the photons can occupy a number of mode as well, which also disrupts the equilibrium. The authors simply figured out how to minimize all these potential issues, leaving the photons in a single state.

So to start with, they brought the mirrors very close together, which favors emissions in a single possible mode. They then used many short bursts from a pump laser to ensure that there was a steady supply of photons to make up for the losses; the pulses were timed so that the system had about eight milliseconds to reach an equilibrium before the next pulse hit. Once the pulses start, the photons scatter off so many excited dye molecules that the entire system rapidly reaches thermodynamic equilibrium in a single energy state, setting the stage for a BEC. At this point, the system "is formally equivalent to an ideal gas of massive bosons." Massive, in this case, being 6.7 x 10-36kg.

Given the physics of the device, the researchers were able to calculate just how many photons they needed in the system to produce a BEC: at room temperature, about 77,000.

At lower photon densities, the light in the device appears as a broad, flat peak across wavelengths near dye's emission wavelength. But as the photon density was cranked up, the device reached what the authors call a thermal equilibrium, where the photons and excited dye molecules reached a sort of steady state. A sharp peak developed right at that wavelength, which matched nicely with calculations based on theoretical considerations.

The distribution of the light also changed, with a sharp point appearing right at the center of the mirror. This sharp, centered peak appeared even if the pump laser was targeted off-center. According to the authors, the off-center pump would put too few photons at the center of the trap if it weren't for things being at an equilibrium. "This effect is not known in lasers," they note.

The authors say there are three things that make them think that they had produced a BEC of photons: the match of the light curves with theory, the phase transition that became apparent as the photon number went up, and the concentration of photons at the center of the mirrored trap. They're not entirely sure what to do with it at the moment, but suggest it has enough properties that are distinct from a standard laser that some use might be figured out. In any case, the work is a nice demonstration of how theoretical predictions, even when really counterintuitive, can produce some interesting effects.

Nature, 2010. DOI: 10.1038/nature09567 (About DOIs).

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