Quantum mechanics

Quantum computer

Cavity quantum electrodynamics

Embedded system

Atomic physics

Excitation

Computer science

Nanophotonics

Quantum optics

Photonics

Photon

Quantum sensor

Physics

Quantum entanglement

Waveguide

Quantum network

Quantum information science

Quantum technology

Atom (system on chip)

Quantum

Optoelectronics

Open quantum system

Nature

Considerable efforts have been recently devoted to combining ultracold atoms and nanophotonic devices1-4 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 nanofibre6,7, we observe a single collective atomic excitation8,9 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 physics10,11.

Waveguide-coupled single collective excitation of atomic arrays