Physicists in the US have come up with a way of making photons repel each other by sending them through an ultracold atomic gas. This astonishing feat could lead to the creation of “photon crystals” and exotic quantum states such as a Mott insulator.   Image caption: In vacuum optical system for photon-photon interactions.

Photons, the smallest energy carriers of light, interact extremely weakly in vacuum. However, realizing strongly interacting photons at the individual photon level is fascinating, as it allows people to use light to control light. This opens the possibility to implement quantum information science, to design all-optical quantum devices, and to form novel quantum many-body states...