  Considerable efforts have been recently devoted to combining ultracold atoms
and nanophotonic devices to obtain not only better scalability and figures of
merit than in free-space implementations, but also new paradigms for
atom-photon interactions. 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, we observe a
single collective atomic excitation 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.
