A single-photon server with just one atom (original) (raw)

Neutral atoms are ideal objects for the deterministic processing of quantum information. Entanglement operations have been carried out by photon exchange 1 or controlled collisions 2 , and atom-photon interfaces have been realized with single atoms in free space 3,4 or strongly coupled to an optical cavity 5,6. A long-standing challenge with neutral atoms, however, is to overcome the limited observation time. Without exception, quantum effects appeared only after ensemble averaging. Here, we report on a single-photon source with one, and only one, atom quasi-permanently coupled to a high-finesse cavity. 'Quasipermanent' refers to our ability to keep the atom long enough to, first, quantify the photon-emission statistics and, second, guarantee the subsequent performance as a single-photon server delivering up to 300,000 photons for up to 30 s. This is achieved by a unique combination of single-photon generation and atom cooling 7-9. Our scheme brings deterministic protocols of quantum information science with light and matter 10-16 closer to realization. Deterministic single-photon sources are of prime importance in quantum information science 17. Such sources have been realized with neutral atoms, embedded molecules, trapped ions, quantum dots and defect centres 18. All of these sources are suitable for applications where the indivisibility of the emitted light pulses is essential. For quantum computing or quantum networking, the emitted photons must also be indistinguishable. Such photons have so far only been produced with quantum dots 19 and atoms 20,21. Another requirement is a high efficiency. This is hard to obtain in free space, as the light-collecting lens covers only a fraction of the full 4π solid angle. The efficiency can be boosted by strongly coupling the radiating object to an optical microcavity, as has been achieved with atoms 5,6 and quantum dots 22. An additional advantage of the cavity is that a vacuum-stimulated Raman adiabatic passage can be driven in a multilevel atom 6,23,24. In this way, the amplitude 5,24 , frequency 20 and polarization 25 of the photon can be controlled. It should also be possible to combine partial photon production with internal atomic rotations for the construction of entangled photon states such as W and GHZ states 15. All of these demands together have so far only been achieved with atoms in high-finesse microcavities. One reason is that neutral atoms are largely immune to perturbations, such as electric patch fields close to dielectric mirrors. However, atomic systems have always suffered from a fast atom loss. We have now implemented a cavity-based scheme, see Fig. 1, with a dipole laser for trapping, a trigger laser for photon generation and a recycling laser for Beam splitter Detector 1 Trigger and recycling laser Rb atom Cavity Dipole trap Detector 2