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 (cQED) has been the subject of spectacular experimental advances, and great efforts have been made to control the coupling rate by trapping 3,4 and cooling the atom 5,6 towards the motional ground state, which has been achieved in one dimension so far 5 . For N atoms, the three-dimensional ground state of motion is routinely achieved in atomic Bose-Einstein condensates (BECs) 7 , but although first experiments combining BECs and optical cavities have been reported recently 8,9 , coupling BECs to strong-coupling cavities has remained an elusive goal. Here we report such an experiment, which is made possible by combining a new type of fibre-based cavity 10 with atom chip technology 11 . This allows single-atom cQED experiments with a simplified setup and realizes the new situation of N atoms in a cavity each of which is identically and strongly coupled to the cavity mode 12 . Moreover, the BEC can be positioned deterministically anywhere within the cavity and localized entirely within a single antinode of the standing-wave cavity field. This gives rise to a controlled, tunable coupling rate, as we confirm experimentally. 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. The system is suitable as a light-matter quantum interface for quantum information 13 . 2 The interaction of an ensemble of N atoms with a single mode of radiation has been a recurrent theme in quantum optics at least since the work of Dicke 14 , who showed that under certain conditions the atoms interact with the radiation collectively, giving rise to new effects such as superradiance. Recently, collective interactions with weak fields, with and without a cavity, have become a focus of theoretical and experimental investigations, especially since it became clear that they can turn the ensemble into a quantum memory 13,15 . Such a memory would become a key element for processing quantum information 13,16 if realized with near-unit conversion efficiency and long storage time. The figure of merit determining the probability of converting an atomic excitation into a cavity photon (a "memory qubit" into a "flying qubit") is the between the ensemble and the field, 2 is the cavity photon decay rate and 2 the atomic spontaneous emission rate. (Up to a factor of order 1, C N is the single-pass optical depth of the atomic sample multiplied by the cavity finesse .) For weak excitation, to which we restrict oursel...
A measurement necessarily changes the quantum state being measured, a phenomenon known as back-action. Real measurements, however, almost always cause a much stronger back-action than is required by the laws of quantum mechanics. Quantum non-demolition measurements have been devised that keep the additional back-action entirely within observables other than the one being measured. However, this back-action on other observables often imposes its own constraints. In particular, free-space optical detection methods for single atoms and ions (such as the shelving technique, a sensitive and well-developed method) inevitably require spontaneous scattering, even in the dispersive regime. This causes irreversible energy exchange (heating), which is a limitation in atom-based quantum information processing, where it obviates straightforward reuse of the qubit. No such energy exchange is required by quantum mechanics. Here we experimentally demonstrate optical detection of an atomic qubit with significantly less than one spontaneous scattering event. We measure the transmission and reflection of an optical cavity containing the atom. In addition to the qubit detection itself, we quantitatively measure how much spontaneous scattering has occurred. This allows us to relate the information gained to the amount of spontaneous emission, and we obtain a detection error below 10 per cent while scattering less than 0.2 photons on average. Furthermore, we perform a quantum Zeno-type experiment to quantify the measurement back-action, and find that every incident photon leads to an almost complete state collapse. Together, these results constitute a full experimental characterization of a quantum measurement in the 'energy exchange-free' regime below a single spontaneous emission event. Besides its fundamental interest, this approach could significantly simplify proposed neutral-atom quantum computation schemes, and may enable sensitive detection of molecules and atoms lacking closed transitions.
We prepare and detect the hyperfine state of a single 87Rb atom coupled to a fiber-based high-finesse cavity on an atom chip. The atom is extracted from a Bose-Einstein condensate and trapped at the maximum of the cavity field, resulting in a reproducibly strong atom-cavity coupling. We use the cavity reflection and transmission signal to infer the atomic hyperfine state with a fidelity exceeding 99.92% in a readout time of 100 μs. The atom is still trapped after detection.
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