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Single photons from many atoms

We report on an experiment to conditionally generate single-photon pulses following the scheme proposed by Duan, Lukin, Cirac, and Zoller (Nature 414, 413 (2001)), but implemented with a sample of laser-cooled atoms inside an optical resonator. A sample of laser-cooled atoms is prepared inside a macroscopic, moderate-finesse optical resonator with a finesse of 1000, and a single-atom cooperativity parameter of 5×10-3 (see Fig. 1). The sample is initialized by optically pumping all atoms into a single magnetic and hyperfine sublevel. Subsequently a pump field (write pump) is applied that weakly drives a spontaneous Raman transition, such that the emission frequency is resonant with the optical cavity. The write pump is sufficiently weak, such that on average much less than one photon (write photon) is emitted into the resonator, and detected with a photon-counting module. If a write photon is detected, the sample that emitted the photon is prepared in an entangled Dicke state, where a single excitation is shared symmetrically between all atoms. Within the coherence time of the Dicke state, the sample can subsequently be made to emit a single photon on demand (read photon) by applying a read pump on a Raman transition where the initial and target states are reversed compared to the write process. The mode of the read photon is precisely defined by a phase-conjugation condition between the four electromagnetic fields involved in the process. We achieve conditional single-photon efficiency above 50% for Raman transitions between hyperfine sublevels, and efficiency above 70% for Raman transitions between magnetic sublevels. The coherence time of the Dicke state is limited by the Doppler effect to values near 2 s. We suggest possibilities to lengthen the coherence time, further improve the source efficiency, and discuss prospects for long-distance entanglement generation between atomic samples.