Electromagnetically-induced transparency (EIT) and the associated appearance of hybrid quasi-particles (dark-state polaritons) in ultracold Rydberg gases have opened intriguing perspectives to create new atom-light interfaces operating at the quantum level and with fully tunable interactions.
For the first time we give a complete picture of Rydberg interacting dark state polaritons by probing both the photonic and atomic degrees of freedom in a single experiment. Strong long-range interactions between Rydberg atoms give rise to an effective interaction blockade for dark-state polaritons, which results in large optical nonlinearities and modified polariton number statistics. Our work provides a better understanding of strongly-interacting dark-state polaritons and creates new avenues in the area of single photon nonlinear optics and for the generation of nonclassical states of light and matter.