QUROPE Quantum Information Processing and Communication in Europe

In conventional nonlinear optics, linear quantum optics1, 2, and cavity quantum electrodynamics3, 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 optics5, 6, 7, 8, 9, 10, optical photons are coherently and reversibly mapped onto collective atomic Rydberg excitations11, giving rise to dipole-mediated effective photon–photon interactions that are long range12, 13. Consequently, a spatial overlap between the light modes is 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 neighbour7, 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 interactions16 are relevant for scalable multichannel photonic devices15, 17 and the study of strongly correlated many-body dynamics using light18.