Contactless nonlinear optics mediated by long-range Rydberg interactions

In conventional nonlinear optics, linear quantum optics(1,2), and cavity quantum electrodynamics(3,4) to create effective photon-photon interactions photons must have, at one time, interacted with matter inside a common medium. In contrast, in Rydberg quantum optics(5-10), optical photons are coherently and reversibly mapped onto collective atomic Rydberg excitations(11), giving rise to dipole-mediated effective photon-photon interactions that are long range(12,13). Consequently, a spatial overlap between the lightmodesis no longer required. We demonstrate such a contactless coupling between photons stored as collective Rydberg excitations in spatially separate optical media. The potential induced by each photon modifies the retrieval mode of its neighbour(7,9,14,15), leading to correlations between them. We measure these correlations as a function of interaction strength, distance and storage time, demonstrating an effective interaction between photons separated by 15 times their wavelength. Contactless effective photon-photon interactions(16) are relevant for scalable multichannel photonic devices(15,17) and the study of strongly correlated many-body dynamics using light(18).