We describe a simple experimental technique which allows us to store a small and deterministic number of neutral atoms in an optical dipole trap. The desired atom number is prepared in a magneto-optical trap overlapped with a single focused Nd:YAG laser beam. Dipole trap loading efficiency of 100 % and storage times of about one minute have been achieved. We have also prepared atoms in a certain hyperfine state and demonstrated the feasibility of a state-selective detection via resonance fluorescence at the level of a few neutral atoms. A spin relaxation time of the polarized sample of 4.2 ± 0.7 s has been measured. Possible applications are briefly discussed. 32.80Pj, 42.50VkNeutral atoms can conveniently and at very low kinetic energy be stored in a magneto-optical trap (MOT) [1], not only in large quantities but also in small and exactly known numbers of up to 20 single atoms [2,3]. For several applications, for instance in cavity quantum electrodynamics [4], it is of interest to use perfectly controlled or deterministic samples of atoms for further experiments involving quantum interactions of an exactly known number of atoms. Full control of all internal and external atomic degrees of freedom is necessary in such applications, but cannot be achieved in a MOT since due to its dissipative character all degrees of freedom are intimately mixed. In order to overcome this problem one can therefore combine the operating convenience of the MOT for isolated atoms with the advantages for quantum manipulation offered by the nearly conservative potential of optical dipole traps. The interest in optical dipole traps [5] as an elegant and simple way to store laser-cooled neutral atoms has rapidly increased within the last few years [6]. Far-off-resonance optical dipole traps [7] can confine atoms in all ground states for a long time with a very small ground-state relaxation rate [8]. Cold atoms can be localized in micropotentials of a 3D periodic potential created by interfering laser beams (so-called optical lattices [9]). Such experiments always operate with at least several thousands of atoms, the atom number can not be determined exactly and is controlled only on average.In the present work we load a small and exactly known atom number into an optical dipole trap with 100% efficiency, opening up a route to a novel kind of cold atom sources free of the indeterminism intrinsic to usual sources like atom beams. We have also demonstrated the feasibility of a state-selective detection at the level of a few neutral atoms and measured a long spin relaxation time of some seconds. Together with recently demonstrated Raman sideband cooling [10] and the generation of non-classical motional states of atoms in standing-wave dipole traps [11], this system promises to be a new basis for future experiments with full control of all atomic degrees of freedom. One of the most interesting possibilities would be long-time localization of more than one atom within a mode of a high finesse cavity.The relevant part of the apparatus is shown...

We have identified and photographed individual cesium atoms in a magnetooptical trap with steep magnetic gradients. By switching off the trapping light fields, single atoms were released to a bound state of the magnetic potential. A storage time of 38 s was measured for purely magnetic trapping, whereas a storage time of 147 s was observed in the corresponding magneto-optical trap.

We have experimentally explored a novel possibility to study exoergic cold atomic collisions. Trapping of small countable atom numbers in a shallow magneto-optical trap and monitoring of their temporal dynamics allows us to directly observe isolated two-body atomic collisions and provides detailed information on loss statistics. A substantial fraction of such cold collisional events has been found to result in the loss of one atom only. We have also observed for the first time a strong optical suppression of ground-state hyperfine-changing collisions in the trap by its repump laser field. 32.80.Lg, 32.80.Pj, 34.50.Rk, 42.50.Vk

We have studied a single neutral atom stored in a magneto-optical trap by recording arrival times of fluorescence photons emitted by the atom. Photon correlations at nanosecond scales ͑Rabi oscillations͒, at microseconds ͑intensity and polarization correlations͒, and also at milliseconds ͑position correlations͒ reveal the dynamical behavior of the atomic excitation, of the atomic orientation, and of its transport in the trap at both the optical wavelength scale and the trap size. ͓S1050-2947͑98͒50109-6͔

We present a detailed analysis of the cold collision measurements performed in a high-gradient magneto-optical trap with a few trapped Cs atoms first presented in Ueberholz et al (J. Phys. B: At. Mol. Opt. Phys. 33 (2000) L135). The ability to observe individual loss events allows us to identify two-body collisions that lead to the escape of only one of the colliding atoms (up to 10% of all collisional losses). Possible origins of these events are discussed here. We also observed strong modifications of the total loss rate with variations in the repumping laser intensity. This is explained by a simple semiclassical model based on optical suppression of hyperfine-changing collisions between ground-state atoms.