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Physical Review Letters

Suppression of Ultracold Ground-State Hyperfine-Changing Collisions with Laser Light

V. Sánchez-Villicaña, S. D. Gensemer, K. Y. N. Tan, Anantharaman Kumarakrishnan, T. Dinneen, Wenko Süptitz, P. L. Gould

Published Jun 5, 1995• 36 citations
Publisher

Abstract

Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine-changing collisions between ultracold ${}^{87}\mathrm{Rb}$ atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\ensuremath{\sim}50%$ and are in reasonable agreement with a simple Landau-Zener model. Our ability to control collisional trap loss processes may have important implications for the achievement of high densities in laser cooled samples.

DOI
10.1103/physrevlett.74.4619

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Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine-changing collisions between ultracold 87Rb{}^{87}\mathrm{Rb}87Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to \ensuremath∼50\ensuremath{\sim}50%\ensuremath∼50 and are in reasonable agreement with a simple Landau-Zener model. Our ability to control collisional trap loss processes may have important implications for the achievement of high densities in laser cooled samples.
Measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms are presented. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below $\\sim$70 mu\\mumuK. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.^ We report on measurements of trap loss collisions between ultracold Rb atoms which are confined in a magneto-optical trap. Both isotopes sp85\\sp{85}sp85Rb and sp87\\sp{87}sp87Rb have been studied over wide range of trap laser intensities and detunings. The trap loss collision rates exhibit variations over 3 orders of magnitude. We observe significant differences between the isotopes. At large detunings the ability of the trap to recapture products of the hyperfine-changing collisions is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room temperature background gas have also been measured.^ Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine changing collisions between ultracold sp87\\sp{87}sp87Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\\sim$50% and are in reasonable agreement with a simple Landau-Zener model.^ Finally, we have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of $\\sim$3 have been observed. ^
We report the observation of hyperfine-changing collisions and the suppression of their rates in a sample of ultracold sodium atoms. We show that the suppression scheme used in the study of photoassociative ionization of Na [Phys. Rev. Lett. 76, 2033 (1996)] is also able to suppress the hyperfine-changing collisions, producing suppression up to 80%. The experimental results are compared with predictions of a simple Landau-Zener model and of a three-dimensional, multichannel close-coupling calculation. Both theories agree at low intensity, but at high intensity the experiment shows a saturation behavior that is qualitatively described only by the three-dimensional multichannel theory.
We present a method to control the cold collision between an ultracold atom and a trapped ion. A laser is used to excite the ground state atom to a repulsive Rydberg potential level at a certain atom-ion distance. In this way the ion is effectively surrounded by a potential wall that the atom cannot cross. Once the atom leaves the interaction area, it is de-excited back to its original level. The adiabaticity of the scheme is analyzed as a function of different parameters such as laser frequency, laser power, initial atom-ion collision energy, as well as the direction of the collisional process with respect to the light field. By controlling e.g. the laser power and the laser frequency, as well as by addressing different Rydberg states, the properties of this shielding effect can be widely tuned. In particular, unwanted chemical reactions between atoms and ion can efficiently be suppressed, which is an important step towards realization of diverse quantum technological applications for hybrid atom-ion systems.
Abstract Understanding ultracold collisions involving molecules is of fundamental importance for current experiments, where inelastic collisions typically limit the lifetime of molecular ensembles in optical traps. Here we present a broad study of optically trapped ultracold RbCs molecules in collisions with one another, in reactive collisions with Rb atoms, and in nonreactive collisions with Cs atoms. For experiments with RbCs alone, we show that by modulating the intensity of the optical trap, such that the molecules spend 75% of each modulation cycle in the dark, we partially suppress collisional loss of the molecules. This is evidence for optical excitation of molecule pairs mediated via sticky collisions. We find that the suppression is less effective for molecules not prepared in the spin-stretched hyperfine ground state. This may be due either to longer lifetimes for complexes in the dark or to laser-free decay pathways. For atom–molecule mixtures, RbCs + Rb and RbCs + Cs, we demonstrate that the rate of collisional loss of molecules scales linearly with the density of atoms. This indicates that, in both cases, the loss of molecules is rate-limited by two-body atom–molecule processes. For both mixtures, we measure loss rates that are below the thermally averaged universal limit.
Near-resonant light is used to modify the collision dynamics of magneto-optically trapped metastable xenon atoms. Enhanced collisional ionization occurs for a "control laser" tuned below resonance, greatly exceeding the predictions of existing models of trap loss very close to resonance. With the trapping laser off, control light tuned above resonance suppresses ionization by a factor of 8. With the trap light on, a suppression factor >30 is observed. Increases in the number and density of trapped atoms and in the trap lifetime attest to the utility of optical control of collisions.
We report on the dynamics of ultracold collisions induced by near-resonant frequency-chirped light. A series of identical chirped pulses, separated by a variable delay, is applied to an ultracold sample of 85Rb^{85}\mathrm{Rb}85Rb, and the rate of inelastic trap-loss collisions is measured. For small detunings of the chirped light below the atomic resonance, we observe that the rate of collisions induced by a given pulse can be increased by the presence of an earlier pulse. We attribute this to the enhancement of short-range collisional flux by the long-range excitation of atom pairs to an attractive molecular potential. For larger detunings and short delays, we find that a leading pulse can suppress the rate of collisions caused by a following pulse. This is due to a depletion of short-range atom pairs by the earlier pulse. Comparison of our data to classical Monte Carlo simulations of the collisions yields reasonable agreement.