An optical cavity enhances the interaction between atoms and light, and the rate of coherent atom–photon coupling can be made larger than all decoherence rates of the system. For single atoms, this ‘strong coupling regime’ of cavity quantum electrodynamics1,2 has been the subject of many experimental advances. Efforts have been made to control the coupling rate by trapping3,4 the atom and cooling5,6 it towards the motional ground state; the latter has been achieved in one dimension so far5. For systems of many atoms, the three-dimensional ground state of motion is routinely achieved7 in atomic Bose–Einstein condensates (BECs). Although experiments combining BECs and optical cavities have been reported recently8,9, coupling BECs to cavities that are in the strong-coupling regime for single atoms has remained an elusive goal. Here we report such an experiment, made possible by combining a fibre-based cavity10 with atom-chip technology11. This enables single-atom cavity quantum electrodynamics experiments with a simplified set-up and realizes the situation of many atoms in a cavity, each of which is identically and strongly coupled to the cavity mode12. Moreover, the BEC can be positioned deterministically anywhere within the cavity and localized entirely within a single antinode of the standing-wave cavity field; we demonstrate that this gives rise to a controlled, tunable coupling rate. We study the heating rate caused by a cavity transmission measurement as a function of the coupling rate and find no measurable heating for strongly coupled BECs. The spectrum of the coupled atoms–cavity system, which we map out over a wide range of atom numbers and cavity–atom detunings, shows vacuum Rabi splittings exceeding 20 gigahertz, as well as an unpredicted additional splitting, which we attribute to the atomic hyperfine structure. We anticipate that the system will be suitable as a light–matter quantum interface for quantum information13.
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We thank J. Hare and F. Orucevic for support in producing the fibre mirror surfaces, and F. Gerbier for the calculation of condensate size in the crossover regime. We acknowledge discussions with Y. Castin and J. Dalibard about atom–light interaction in BECs, as well as with T. W. Hänsch, I. Cirac, P. Treutlein and R. Long. This work was supported by a European Young Investigator Award (EURYI), a Chaire d’Excellence of the French Ministry for Research, and by the EU (‘Atom Chips’ Research Training Network and ‘SCALA’ Integrated Programme). The Atom Chip team at Laboratoire Kastler Brossel is part of the Institut Francilien de Recherche sur les Atomes Froids (IFRAF).
This file contains Supplementary Notes with additional information on collective atom-field interaction and describes two models that we refer to in the letter: the multilevel coupling model that predicts an anticrossing in the vacuum-Rabi spectrum, and the momentum-diffusion model for cavity field-induced heating of the atom cloud.
About this article
Applied Optics (2019)