Neutral atoms are ideal objects for the deterministic processing of quantum information. Entanglement operations have been performed by photon exchange [1] or controlled collisions.[2] Atom-photon interfaces were 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-onlyone atom quasi permanently coupled to a high-finesse cavity. Quasi permanent 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 seconds. This is achieved by a unique combination of single-photon generation and atom cooling. [7,8,9] Our scheme brings truly deterministic protocols of quantum information science with light and matter [10,11,12,13,14,15,16] 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 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. [23] In this way amplitude, [5,24] frequency, [20] and polarization [25] of the photon can be controlled. It should also make 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 these demands together have so far only been achieved with atoms in high-finesse microcavities. A 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 figure 1, with a dipole laser for trapping, a trigger laser for photon generation and a recycling laser for repumping, monitoring and cooling the atom. [8,9] The scheme combines high photon-generation efficiency and long trapping times. The most remarkable features are, first, that the single-photon stream is specified by its intensity correlation function evaluated in realtime during a short time int...

Taming quantum dynamical processes is the key to novel applications of quantum physics, e.g. in quantum information science. The control of light-matter interactions at the single-atom and single-photon level can be achieved in cavity quantum electrodynamics, in particular in the regime of strong coupling where atom and cavity form a single entity. In the optical domain, this requires permanent trapping and cooling of an atom in a micro-cavity. We have now realized three-dimensional cavity cooling and trapping for an orthogonal arrangement of cooling laser, trap laser and cavity vacuum. This leads to average single-atom trapping times exceeding 15 seconds, unprecedented for a strongly coupled atom under permanent observation.Comment: 4 pages, 4 figure

The coupling of individual atoms to a high-finesse optical cavity is precisely controlled and adjusted using a standing-wave dipole-force trap, a challenge for strong atom-cavity coupling. Ultracold Rubidium atoms are first loaded into potential minima of the dipole trap in the center of the cavity. Then we use the trap as a conveyor belt that we set into motion perpendicular to the cavity axis. This allows us to repetitively move atoms out of and back into the cavity mode with a repositioning precision of 135 nm. This makes possible to either selectively address one atom of a string of atoms by the cavity, or to simultaneously couple two precisely separated atoms to a higher mode of the cavity.

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