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# Waveguide-coupled single collective excitation of atomic arrays

## Abstract

Considerable efforts have been recently devoted to combining ultracold atoms and nanophotonic devices1,2,3,4 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 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.

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## Data availability

The data that support the findings of this study are available from the corresponding author on request.

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## Acknowledgements

This work was supported by the European Research Council (Starting grant HybridNet), the Emergence programme from Ville de Paris (project NanoQIP), the DIM Nano-K from Région Ile-de-France, and the PERSU programme from Sorbonne Université (ANR-11-IDEX-0004-02). N.V.C. and A.S.S. acknowledge support from the EU (Marie Curie fellowships Nanofi 659337 and NanoArray 705161), and J.L. thanks the Institut Universitaire de France. We also thank D. Maxein, A. Nicolas and O. Morin for their contributions in the early stage of the experiment.

### Reviewer information

Nature thanks D. Chang, M. J. Hartmann and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

N.V.C., J.R. and A.C. performed the experiment. B.G. contributed to the preparation of the setup and A.S.S. to the data analysis. All authors discussed the results and contributed to the writing of the manuscript. J.L. conceived the experiment and supervised its implementation.

### Conflict of interest

The authors declare no competing interests.

Correspondence to Julien Laurat.

## Extended data figures and tables

1. ### Extended Data Fig. 1 Filtering system.

The Field-1 beam travels through a filtering system, where it is separated from the dipole trapping beams and the write-field component coupled to the nanofibre. Two cascaded volume Bragg gratings (VBG) provide isolation from the dipole beams, whereas the combination of a polarizing beam splitter (PBS) and a lens cavity (LC) provides the desired isolation from the write beam. The total transmission of the filtering system is around 40%, including fibre coupling. A similar filtering system is used for Field 2.

2. ### Extended Data Fig. 2 Timing diagram of the experiment.

After the elongated MOT is loaded, the magnetic field (B) gradient is turned off and the dipole trap is loaded by a transient molasses stage, in which the MOT detuning is increased while the MOT power, as well as the MOT repumping power, are decreased. An additional repumping pulse at the end of the molasses stage is sent to the atoms. After this point, the writing-and-retrieval process starts. The dipole-trapping beams are always on. The full cycle is performed at a repetition rate of 10 Hz.

3. ### Extended Data Fig. 3 Dipole trap.

a, Probe transmission (T) spectra as a function of the detuning Δ from resonance for the transition $$\left|6{{\rm{S}}}_{1/2},F=4\right\rangle \to \left|6{{\rm{P}}}_{3/2},F^{\prime} =5\right\rangle$$. The fits are given by the function exp[−OD/(1 + (2Δ/Γ)2)], with OD = 97 ± 2 (red), OD = 1.4 ± 0.1 (black) and Γ/(2π) = 5.8 ± 0.2 MHz. b, Decay of the absorption after dipole trap loading. The exponential decay (green solid line) leads to a 25-ms lifetime. The error bars correspond to the propagated Poissonian error of the photon counting probabilities.

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• #### DOI

https://doi.org/10.1038/s41586-019-0902-3