With great interest I read an article about a paper where scientists were able to create two photon bound states ("molecules of light"). http://physicsworld.com/cws/article/news/2013/sep/26/physicists-create-molecules-of-light I was quite astonished since light normally does not self-interact (apart from Delbrück scattering, which is strictly speaking no direct interaction). Of course, in this experiment the interaction between the two photons is not direct either, but mediated via the interaction of the photons with the electrons in the ultracold Rubidium atoms. Still I wonder, how such a two photon bound state is described theoretically and how far one can go with the analogue of a normal molecule: Can one define an average distance between these two photons? It sounds at least strange, since photons are normally not localizable, are they? Are these states photons at all or merely mixed photon-electron states? How long can this bound state in principle live? Are there energy levels such as a electron bound to a nucleus has them?