Bose-Einstein condensate of photons
Normally if you cool a gas the atoms exist in a variety of different energies, but Einstein and Bose predicted that if you cool certain types of atoms enough they start to condense into a single state all with exactly identical energy momentum etc., essentially a new state of matter.
Although it was predicted in 1938 it wasn't achieved until 1995 when Eric Cornell and Carl Wieman cooled a very low pressure rubidium gas to about 170 billionths of a degree above absolute zero and they formed a bose-einstein condensate. This got physicists very interested as it is a fascinating example of a quantum mechanical effect, but at that temperature they could almost never be used for anything practical.
If you used light for this purpose the same thing could happen at much higher temperatures, however the problem is that you can't confine and cool down light, it tends to get absorbed by the walls far too quickly.
Jan Klaers and collegues have however got around this problem, they have trapped photons with two curved mirrors very close together only about 3.5 wavelengths, this means that the photons can only exist at certain wavelengths, and if those can't be absorbed by the walls or medium in between they stay in the cavity, and the photons of light can be cooled by interacting with dye molecules in between the mirrors causing the differences in state between them to slowly tend towards the temperature of the dye molecules. This has produced a Bose Einstein Condensate of photons for the first time, which is pretty impressive but what is more impressive is that they made it at room temperature, so it could be practical to use.
They haven't got to that point yet, but photon Bose Einstein Condensates may be able to produce high quality laser like light at wavelengths that are difficult to make lasers at, such as the ultraviolet, plus other more exotic applications that a new state of matter may open up.