1. Laboratoire Kastler Brossel, ENS/UPMC-Paris 6/CNRS, 24 rue Lhomond, 75005 Paris, France
2. Max-Planck-Institut für Quantenoptik/LMU, Schellingstr. 4, 80799 München, Germany
3. These authors contributed equally to this work.
4. Present address: BMW Group, Abt. Instrumentierung und Displays, Knorrstr. 147, D-80788 München, Germany.

Correspondence to: Jakob Reichel¹ Correspondence and requests for materials should be addressed to J.R. (Email: jakob.reichel@ens.fr).

Abstract

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 electrodynamics^{1, 2} has been the subject of many experimental advances. Efforts have been made to control the coupling rate by trapping^{3, 4} the atom and cooling^{5, 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 recently^{8, 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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