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It is mentioned in this article It seems to be a Bose-Einstein condensate of some kind, but it is not exactly clear how one can create a BEC with just photons

Light consists of tiny indivisible portions, the photons. Under certain conditions, they, too, can condense, if they are cooled enough. Many thousands of these light packets then suddenly fuse into a kind of super-photon with unusual characteristics – a so-called Bose-Einstein condensate.

How do you "cool light"???

Read more at: http://phys.org/news/2016-04-capacity-condensed.html#jCp

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It is not possible to get a Bose-Einstein condensate of photons in three-dimensional equilibrium. Since the photons have no mass gap and no chemical potential, they can just be absorbed by the walls. In this example, however, the experimenters used a gas that was out of equilibrium, with different effective temperatures for the motion in different directions.

They used a cavity to create an effectively two-dimensional photon gas, trapping light between two nearly flat surfaces. In the narrow dimension, normal to the surfaces, the temperature is relatively high, and the experimenters selected out the first TE mode in this direction. However, they cool the photons in the other two dimensions quite a bit more. Looking at the photons moving in the other two directions, they look like a two-dimensional gas of massive particles. The mass corresponds to the energy caught up in the motion in the third dimension. When sufficiently cooled, this two-dimensional gas can look like a condensate.

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