Waveguide-coupled single collective excitation of atomic arrays

Abstract

Considerable efforts have been recently devoted to combining ultracold atoms and nanophotonic devices^1,2,3,4 to obtain not only better scalability and figures of merit than in free-space implementations, but also new paradigms for atom–photon interactions⁵. Dielectric waveguides offer a promising platform for such integration because they enable tight transverse confinement of the propagating light, strong photon–atom coupling in single-pass configurations and potentially long-range atom–atom interactions mediated by the guided photons. However, the preparation of non-classical quantum states in such atom–waveguide interfaces has not yet been realized. Here, by using arrays of individual caesium atoms trapped along an optical nanofibre^6,7, we observe a single collective atomic excitation^8,9 coupled to a nanoscale waveguide. The stored collective entangled state can be efficiently read out with an external laser pulse, leading to on-demand emission of a single photon into the guided mode. We characterize the emitted single photon via the suppression of the two-photon component and confirm the single character of the atomic excitation, which can be retrieved with an efficiency of about 25%. Our results demonstrate a capability that is essential for the emerging field of waveguide quantum electrodynamics, with applications to quantum networking, quantum nonlinear optics and quantum many-body physics^10,11.