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Physical Review A

Reduction of light-assisted collisional loss rate from a low-pressure vapor-cell trap

Michael H. Anderson, Wolfgang H. Petrich, J. R. Ensher, Eric Cornell

Published Nov 1, 1994• 87 citations
Publisher

Abstract

We have demonstrated an order of magnitude increase in the number and in the lifetime of magneto-optically trapped $^{87}\mathrm{Rb}$ atoms collected from a low-pressure vapor. The improvement results from the reduction of light-assisted collision losses and is achieved by shelving the atomic population into the lower hyperfine ground state. Large fractional populations of the lower hyperfine ground state have been obtained by blocking the hyperfine repumping light in the region of the trapped atoms and by application of laser light that optically pumps the atoms into that state. We have observed trap lifetimes in excess of 700 s at our highest vacuum levels. At the rubidium pressure that maximizes the product of the number and lifetime of trapped atoms we observe a lifetime of 240 s with 5\ifmmode\times\else\texttimes\fi{}${10}^{7}$ trapped atoms, under conditions for which an ordinary magneto-optical trap has a lifetime of only 14 s with 3\ifmmode\times\else\texttimes\fi{}${10}^{6}$ atoms.

DOI
10.1103/physreva.50.r3597

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Physical Review A•Dec 29, 2008•20 citations
We have demonstrated an order of magnitude increase in the number and in the lifetime of magneto-optically trapped 87Rb^{87}\mathrm{Rb}87Rb atoms collected from a low-pressure vapor. The improvement results from the reduction of light-assisted collision losses and is achieved by shelving the atomic population into the lower hyperfine ground state. Large fractional populations of the lower hyperfine ground state have been obtained by blocking the hyperfine repumping light in the region of the trapped atoms and by application of laser light that optically pumps the atoms into that state. We have observed trap lifetimes in excess of 700 s at our highest vacuum levels. At the rubidium pressure that maximizes the product of the number and lifetime of trapped atoms we observe a lifetime of 240 s with 5\ifmmode\times\else\texttimes\fi{}107{10}^{7}107 trapped atoms, under conditions for which an ordinary magneto-optical trap has a lifetime of only 14 s with 3\ifmmode\times\else\texttimes\fi{}106{10}^{6}106 atoms.
A detailed experimental and theoretical investigation of a magneto-optical trap for caesium atoms is presented. Particular emphasis has been placed on achieving high spatial number densities and low temperatures. Optimizing both of these together enables efficient evaporative cooling from a conservative trap, a procedure which has recently led to the first observations of Bose-Einstein condensation in a dilute atomic vapour. The behaviour of a magneto-optical trap is nominally determined by four independent parameters: the detuning and intensity of the light field, the magnetic field gradient and the number of trapped atoms. A model is presented which incorporates previous treatments into a single description of the trap that encompasses a wide range of its behaviour. This model was tested quantitatively by measuring the temperature of the cloud and its spatial distribution as a function of the four parameters. The maximum density was found to be limited both by the reabsorption of photons scattered within the cloud and by a reduction of the confining force at small light shifts. The nonlinear variation with position of the restoring force was found to be significant in limiting the number of atoms confined to a high density. A maximum density in phase space (defined as the number of atoms in a box with sides of dimension one thermal de Broglie wavelength) of (1.5 ± 0.5) x 10-5 was observed, with a spatial density of 1.5 x 1011 atoms per cm3. Cold collision losses from a caesium magneto-optical trap have been studied with the purpose of assessing their influence on spatial densities. In contrast to previous measurements of similar quantities, these measurements did not require the use of an ultra-low (< 10-10 Torr) background vapour pressure. The dependence of the cold collision loss coefficient β on the trapping intensity was measured to permit identification of the different cold collision processes. The largest loss rates observed were those due to hyperfine structure-changing collisions, with a coefficient β = (2±1) x 10-10cm3s-1. A study is presented of a modified magneto-optical trap in which a fraction of the population is shelved into a hyperfine level that does not interact with the trapping light. In this so-called "dark" magneto-optical trap, improved densities of nearly 1012cm-3 have been previously reported for sodium. The application of the technique to caesium is not straightforward due to the larger excited state hyperfine splittings. A simple theory for caesium is presented and its main predictions verified by measurements of density, number and temperature. A density of nearly 1012cm,-3 was indeed obtained but at a temperature substantially higher than in the conventional magneto-optical trap.
A novel method has been demonstrated for increasing the number of 85Rb trapped atoms in a vapor-cell magneto-optical trap. By using a second set of trapping laser beams overlapped with the original beams and detuned further from resonance, we obtained a doubling of the trap-filling rate as higher initial velocities were slowed. Blocking out the centers of these new beams and reducing the intensity in the original beams also allowed us to obtain a reduction in the light-induced loss rate. Overall we have obtained a fivefold increase in the number of trapped atoms. With 20-mm-diameter beams, ~1.4 × 109 atoms could be trapped. A Gaussian density profile with a peak density of ~2.2 × 1011 cm−3 was observed.
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 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.
In a ceramic vapor cell we have created a robust Sr magneto- optical trap that stores about 108 atoms with lifetimes > 200 ms. We eliminate the 5p 1P1 yields 4d1D2 yields 5p 3P2 leak and achieve a 10-fold improvement in trap lifetime by re-pumping the 5p 3P0,2 dark states with 679 nm and 707 nm light. The observed lifetime is now limited by cold collision losses, and we have preliminary measurements of the 2-body loss rate. Direct readout of the trap velocity distribution is possible using the narrow 5s2 1S0 yields 5p 3P1 intercombination line at 689 nm. We can also cool with this narrow transition and have achieved a 40-fold 1D velocity compression for about 5 percent of the trapped atoms by applying this second-stage cooling.
We report cooling and trapping of 85Rb^{85}\mathrm{Rb}85Rb atoms in the lower hyperfine level F=2F=2F=2 of the 52S1∕25\phantom{\rule{0.3em}{0ex}}^{2}S_{1∕2}52S1∕2​ ground state in a magneto-optical trap. A maximum of $\ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms could be trapped using trap laser frequency red detuned by 12MHz12\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}12MHz from the $F=2\ensuremath{\rightarrow}{F}^{\ensuremath{'}}=1$ cycling transition. The loading behavior of the trap in terms of loading rate, collisional loss rate, number and density of trapped atoms was studied as a function of trap laser intensity.