Quantum technologies

We have sympathetically cooled a small sample of 133Cs atoms with 87Rb to below 1 μK. Evaporative cooling was realized with microwave radiation driving the Rb ground-state hyperfine transition. By analysing the sympathetic cooling dynamics, we derive a lower limit of the modulus of the Rb–Cs interspecies triplet s-wave scattering length of 200 a_0. For temperatures below 5 μK we observe strong non-exponential losses of the Cs sample in the presence of the Rb sample.

We present a detailed analysis of the cold collision measurements performed in a high-gradient magneto-optical trapwith a few trapped Cs atomsfirst 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.

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 $4.2\pm 0.7$ s has been measured. Possible applications are briefly discussed.

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.

In dieser Arbeit wurde die Verwirklichung einer Dipolfalle für wenige und einzelne Atome vorgestellt. Die gezielte Präparation von kleinen atomaren Ensembles oder einzelnen Teilchen stellt eine kritische Anforderung auf dem Weg zur Erzeugung von verschränkten Zuständen dar, und die aufgebaute Dipolfalle ist der erste Schritt dahin. Von den vorgestellten Messergebnissen sind die langen Speicherzeiten besonders hervorzuheben. Es wurde gezeigt, daß die erreichbaren mittleren Speicherzeiten der einzelnen Atome nur vom Hintergrundgasdruck abhängen und somit sehr lange Speicherzeiten um die 50 s erreichbar sind. Darüber hinaus wird man durch die 100% Effizienz beim Umladen von der MOT in die Dipolfalle sowie die zweifelsfreie Bestimmung der Atomanzahl in der MOT in die Lage versetzt, die Dipolfalle deterministisch mit einer beliebigen Anzahl von Atomen zu laden. Zusammen mit den Ergebnissen der zustandsselektiven Spektroskopie verfügt man nun über eine Apparatur, die eine gewünschte Anzahl von Atomen in einen bestimmten Hyperfeinzustand präparieren kann. Insbesondere ist die Relaxationszeit für die Verteilung der Hyperfeinzustände signifikant größer als die Anregungszeit durch den VAG-Laser, in Übereinstimmung mit der Theorie.

Information on the dynamics of a single neutral atom can be decoded from fluctuations in the resonance fluorescence. We have measured two-time photon correlations of individual cesium atoms stored in a magneto-optical trap. We observe strong correlations at nanosecond scales (Rabi oscillations), at microseconds (intensity and polarization correlations), and also at milliscconds (position correlations) revealing 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. In this article we compare our experimental results with a simplified model of an atom moving through an optical lattice. We investigate the influence of light-field topogaphy and of the multilevel character of the atom on the shape and the visibility of the correlations.