The creation of a photon-atom bound state was first envisaged for the case of an atom in a long-lived excited state inside a high-quality microwave cavity. In practice, however, light forces in the microwave domain are insufficient to support an atom against gravity. Although optical photons can provide forces of the required magnitude, atomic decay rates and cavity losses are larger too, and so the atom-cavity system must be continually excited by an external laser. Such an approach also permits continuous observation of the atom's position, by monitoring the light transmitted through the cavity. The dual role of photons in this system distinguishes it from other single-atom experiments such as those using magneto-optical traps, ion traps or a far-off-resonance optical trap. Here we report high-finesse optical cavity experiments in which the change in transmission induced by a single slow atom approaching the cavity triggers an external feedback switch which traps the atom in a light field containing about one photon on average. The oscillatory motion of the trapped atom induces oscillations in the transmitted light intensity; we attribute periodic structure in intensity-correlation-function data to 'long-distance' flights of the atom between different anti-nodes of the standing-wave in the cavity. The system should facilitate investigations of the dynamics of single quantum objects and may find future applications in quantum information processing.

All conventional methods to laser-cool atoms rely on repeated cycles of optical pumping and spontaneous emission of a photon by the atom. Spontaneous emission in a random direction provides the dissipative mechanism required to remove entropy from the atom. However, alternative cooling methods have been proposed for a single atom strongly coupled to a high-finesse cavity; the role of spontaneous emission is replaced by the escape of a photon from the cavity. Application of such cooling schemes would improve the performance of atom-cavity systems for quantum information processing. Furthermore, as cavity cooling does not rely on spontaneous emission, it can be applied to systems that cannot be laser-cooled by conventional methods; these include molecules (which do not have a closed transition) and collective excitations of Bose condensates, which are destroyed by randomly directed recoil kicks. Here we demonstrate cavity cooling of single rubidium atoms stored in an intracavity dipole trap. The cooling mechanism results in extended storage times and improved localization of atoms. We estimate that the observed cooling rate is at least five times larger than that produced by free-space cooling methods, for comparable excitation of the atom.

The Stark interaction of polar molecules with an inhomogeneous electric field is exploited to select slow molecules from a room-temperature reservoir and guide them into an ultrahigh vacuum chamber. A linear electrostatic quadrupole with a curved section selects molecules with small transverse and longitudinal velocities. The source is tested with formaldehyde (H 2 CO) and deuterated ammonia (ND 3 ). With H 2 CO a continuous flux is measured of Ϸ10 9 /s and a longitudinal temperature of a few kelvin. The data are compared with the result of a Monte Carlo simulation.The past years have seen an explosion of activity in the field of cold atomic gases ͓1͔. It is interesting and desirable to extend these investigations to molecules, which have a complex internal structure and can as a consequence possess a permanent electric dipole moment. Trapping cold polar molecules will lead to new physics due to the long range and anisotropy of the dipole-dipole interaction ͓2͔. Slow molecules for precision measurements or interferometry are further motivations behind the ongoing efforts. However, the complexity and density of energy levels in the rotational and vibrational manifolds largely precludes the effective use of laser cooling techniques ͓3͔. Therefore, a number of different approaches have been considered for cooling and trapping molecules. Buffer-gas cooling in a cryogenic environment is one possibility, but requires a rather complex setup ͓4͔. Another method is photoassociation, but this is limited to simple molecules with laser-cooled precursor atoms ͓5͔. A different technique uses deceleration by the Stark effect, where packages of polar molecules are decelerated with time-varying electric fields ͓6-8͔. Other, mostly mechanical methods have also been proposed but remain to be demonstrated ͓9,10͔.It is, however, not necessary to produce slow molecules, as they are present in any thermal gas, even at room temperature. Slow molecules only need to be filtered out. For this reason, already in the 1950s it was attempted to select the slowest atoms from a hot beam using gravity ͓11͔. These attempts failed, mostly because the slow particles were kicked away by the fast ones. Much later, it was demonstrated that slow lithium atoms can be efficiently guided out of a hot beam with strong permanent magnets, providing a robust and cheap source of slow atoms ͓12͔, e.g., for Bose-Einstein condensation experiments. In the same spirit, an efficient and simple filtering technique could play an important role towards the production of a cold molecular gas.In this paper we describe an experiment in which the Stark interaction of polar molecules with an inhomogeneous, electrostatic field is exploited to efficiently select and guide slow molecules out of a room-temperature reservoir into ultrahigh vacuum. Whether a dipolar molecule is weak-field seeking and trapped by an electric-field minimum, or strongfield seeking and expelled, depends on whether the average orientation of the rotating molecular dipole is antiparallel or parallel to t...

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