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arXiv:1107.5781 · Physical Review A

Light-assisted collisions between a few cold atoms in a microscopic dipole trap

Andreas Fuhrmanek, Ronan Bourgain, Yvan R. P. Sortais, Antoine Browaeys

Published Jun 25, 2012• 59 citations
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Abstract

We study light-assisted collisions in an ensemble containing a small number ($\ensuremath{\sim}$3) of cold ${}^{87}$Rb atoms trapped in a microscopic dipole trap. Using our ability to operate with one atom exactly in the trap, we measure the one-body heating rate associated with a near-resonant laser excitation, and we use this measurement to extract the two-body loss rate associated with light-assisted collisions when a few atoms are present in the trap. Our measurements indicate that the two-body loss rate can reach surprisingly large values $\ensuremath{\beta}>{10}^{\ensuremath{-}8}$ cm${}^{3}$ s${}^{\ensuremath{-}1}$ and varies rapidly with the trap depth and the parameters of the excitation light.

DOI
10.1103/physreva.85.062708

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Bulletin of the American Physical Society•May 1, 1993•0 citations
We study light-assisted collisions in an ensemble containing a small number (\ensuremath∼\ensuremath{\sim}\ensuremath∼3) of cold 87{}^{87}87Rb atoms trapped in a microscopic dipole trap. Using our ability to operate with one atom exactly in the trap, we measure the one-body heating rate associated with a near-resonant laser excitation, and we use this measurement to extract the two-body loss rate associated with light-assisted collisions when a few atoms are present in the trap. Our measurements indicate that the two-body loss rate can reach surprisingly large values \ensuremathβ>10\ensuremath−8\ensuremath{\beta}>{10}^{\ensuremath{-}8}\ensuremathβ>10\ensuremath−8 cm3{}^{3}3 s\ensuremath−1{}^{\ensuremath{-}1}\ensuremath−1 and varies rapidly with the trap depth and the parameters of the excitation light.
We study light-assisted collisions in an ensemble containing a small number (~3) of cold Rb87 atoms trapped in a microscopic dipole trap. Using our ability to operate with one atom exactly in the trap, we measure the one-body heating rate associated to a near-resonant laser excitation, and we use this measurement to extract the two-body loss rate associated to light-assisted collisions when a few atoms are present in the trap. Our measurements indicate that the two-body loss rate can reach surprisingly large values beta>10^{-8} cm^{3}.s^{-1} and varies rapidly with the trap depth and the parameters of the excitation light.
We investigate experimentally the number statistics of a mesoscopic ensemble of cold atoms in a microscopic dipole trap loaded from a magneto-optical trap and find that the atom-number fluctuations are reduced with respect to a Poisson distribution due to light-assisted two-body collisions. For numbers of atoms $N\ensuremath{\gtrsim}2$, we measure a reduction factor (Fano factor) of $0.72\ifmmode\pm\else\textpm\fi{}0.07$, which differs from 1 by more than four standard deviations. We analyze this fact by a general stochastic model describing the competition between the loading of the trap from a reservoir of cold atoms and multiatom losses, leading to a master equation. Applied to our experimental regime, this model indicates an asymptotic value of 3/43/43/4 for the Fano factor at large NNN and in the steady state. We thus show that we have reached the ultimate level of reduction in number fluctuations in our system.
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 investigate the role of hyperfine structure in resonant-dipole interactions between two atoms cotrapped in an optical tweezer. Two-body loss rates from light-assisted collisions (LACs) are measured across the ^{87}Rb hyperfine spectrum and connected to properties of molecular photoassociation potentials via a semiclassical model. To obtain our results, we introduce an imaging technique that leverages repulsive LACs to detect two atoms in a trap, thereby circumventing parity constraints in tweezers. Our findings offer key insights for exploiting hyperfine structure in laser-induced collisions to control cold atoms and molecules in a broad range of quantum science applications.
We report on measurements of trap-loss collisions between ultracold Rb atoms that are confined in a magneto-optical trap. Both isotopes, 85Rb{}^{85}\mathrm{Rb}85Rb and 87Rb,{}^{87}\mathrm{Rb},87Rb, have been studied over a wide range of trap laser intensities and detunings. The trap-loss collisional rate constant exhibits variations over three orders of magnitude. At low intensities and/or large detunings, the trap loss is dominated by ground-state hyperfine-changing collisions, while at high intensities and small detunings, collisions involving excited atoms are more important. We see significant differences between the isotopes in both regimes. At large detunings, the ability of the trap to recapture products of a hyperfine-changing collision 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. A surprisingly large increase in this rate is seen when the confining power of the trap is reduced.
We have confined up to 10{sup 4} {sup 85}Rb atoms in an optical dipole force trap operating up to 65 nm from atomic resonance. For detunings greater than 16 nm, we obtained long confinement times, limited only by background gas collisions, without additional cooling. However, for trapping laser tunings between 4 nm and 16 nm to the red of the lowest Rb (D1) resonance, we observed a novel trap loss mechanism which results in much shorter-trap lifetimes. The trap loss rate is very sensitive to the laser wavelength, and exhibits a complex spectrum. This trap loss may arise from a photo-associative collisional process in which an initially free pair of colliding ground state atoms is promoted to a bound molecular excited state. The bound excited molecule could then be lost from the trap by photoionization, or by spontaneous decay to an untrapped ground state. The results of our preliminary experiments and of further experiments to clarify this novel trap loss process will be presented.