Repulsive photons in a quantum nonlinear medium

The ability to control strongly interacting light quanta (photons) is of central importance in quantum science and engineering(1-5). Recently it was shown that such strong interactions can be engineered in specially prepared quantum optical systems(6-10). Here, we demonstrate a method for coherent control of strongly interacting photons, extending quantum nonlinear optics into the domain of repulsive photons. This is achieved by coherently coupling photons to several atomic states, including strongly interacting Rydberg levels in a cold Rubidium gas. Using this approach we demonstrate both repulsive and attractive interactions between individual photons and characterize them by the measured two- and three-photon correlation functions. For the repulsive case, we demonstrate signatures of interference and self ordering from three-photon measurements. These observations open a route to study strongly interacting dissipative systems and quantum matter composed of light such as a crystal of individual photons(11,12).