Germany's Bonn University and Cologne University physicists cooled photons down to a 'Bose-Einstein condensate' in order to split photons and collect them in optical "valleys" from which they can't return. Physicists used a Bose-Einstein condensate made of photons, which physicist Martin Weitz achieved in 2010. The photons in the 'valleys' "enter the lowest energy state of the system", Weitz said.