Crystallising photons — light becomes matter

The interactions between atoms and photons (i.e. particles of light) has been investigated for a long time. In recent years it became possible to precisely control them to a high degree. The results are fascinating. In particular, it is possible to employ atoms to mediate strong interactions between photons. As a many-body system, a collection of interacting photons is a very interesting object of research, whose investigation has just scratched the surface of a complex and novel phenomenology. It turns out that under proper conditions the photons can crystallise — light becomes matter.