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arXiv:quant-ph/0602010 · Physical Review A

Optimized production of large Bose-Einstein condensates

D. Comparat, A. Fioretti, Guillaume Stern, Emiliya Dimova, B. Laburthe-Tolra, P. Pillet

Published Apr 18, 2006• 55 citations
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Abstract

We describe several routes to quantum degenerate gases based on simple schemes to efficiently load atoms into and evaporate them from a ``dimple'' crossed dipolar trap. The dimple is loaded nonadiabatically by collisions between atoms which are trapped in a reservoir which can be provided either by a dark spontaneous-force magneto-optical trap (MOT), the (aberrated) laser beam itself, or by a quadrupolar or quadratic magnetic trap. Optimal loading parameters for the dimple, relatively high temperature, and tight optical trap are derived from thermodynamic equations including possible inelastic and Majorana losses. Evaporative cooling is described by a set of simple equations, taking into account gravity, the possible occurrence of the hydrodynamical regime, Feshbach resonances, and three body recombination. The solution implies that to have efficient evaporation the elastic collisional rate (in ${\mathrm{s}}^{\ensuremath{-}1}$) must be on the order of the trap frequency and lower than 100 times the temperature in microkelvins. Following this approach Bose-Einstein condensates with more than ${10}^{7}$ atoms should be obtained in much less than $1\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ starting from an ordinary MOT setup.

DOI
10.1103/physreva.73.043410

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We describe several routes to quantum degenerate gases based on simple schemes to efficiently load atoms into and evaporate them from a ``dimple'' crossed dipolar trap. The dimple is loaded nonadiabatically by collisions between atoms which are trapped in a reservoir which can be provided either by a dark spontaneous-force magneto-optical trap (MOT), the (aberrated) laser beam itself, or by a quadrupolar or quadratic magnetic trap. Optimal loading parameters for the dimple, relatively high temperature, and tight optical trap are derived from thermodynamic equations including possible inelastic and Majorana losses. Evaporative cooling is described by a set of simple equations, taking into account gravity, the possible occurrence of the hydrodynamical regime, Feshbach resonances, and three body recombination. The solution implies that to have efficient evaporation the elastic collisional rate (in s\ensuremath−1{\mathrm{s}}^{\ensuremath{-}1}s\ensuremath−1) must be on the order of the trap frequency and lower than 100 times the temperature in microkelvins. Following this approach Bose-Einstein condensates with more than 107{10}^{7}107 atoms should be obtained in much less than 1s1\phantom{\rule{0.3em}{0ex}}\mathrm{s}1s starting from an ordinary MOT setup.
We reported a simple scheme of producing Bose–Einstein condensate (BEC) in a dimple trap. Starting from a regular magneto-optical trap (MOT), we apply a temporal dark MOT to increase the atom density and directly load atoms into a crossed dipole trap. In order to make the trap frequency relatively high at the end of evaporation, a tight dimple is added. Such a scheme ensures relatively high trap volume for loading atoms and high trap frequency for evaporative cooling. The BEC phase transition is observed after 12 s evaporation. Furthermore, we demonstrated the manipulation of the hyperfine states with a Raman coupling, which is very useful for studying many-body physics via Ramsey or spin-echo spectroscopy.
We report on the realization of a sodium Bose–Einstein condensate (BEC) in a combined red-detuned optical dipole trap, formed by two beams crossing in a horizontal plane and a third, tightly focused dimple trap propagating vertically. We produce a BEC in three main steps: loading of the crossed dipole trap from laser-cooled atoms, an intermediate evaporative cooling stage which results in efficient loading of the auxiliary dimple trap, and a final evaporative cooling stage in the dimple trap. Our protocol is implemented in a compact setup and allows us to reach quantum degeneracy even with relatively modest initial atom numbers and available laser power. 1.
We report on the realization of a sodium Bose–Einstein condensate (BEC) in a combined red-detuned optical dipole trap formed by two beams crossing in a horizontal plane and a third, tightly focused dimple trap (dT) propagating vertically. We produce a BEC in three main steps: loading of the crossed dipole trap from laser-cooled atoms, an intermediate evaporative cooling stage that results in efficient loading of the auxiliary dT, and a final evaporative cooling stage in the dT. Our protocol is implemented in a compact setup and allows us to reach quantum degeneracy even with relatively modest initial atom numbers and available laser power.
Optical trapping techniques and the ability to tune the atomic interactions both have made the unprecedented progress in the quantum gas research field. The major advantage of the optical trap is that the atoms are likely to be trapped at various sub-levels of the electronic ground state and the interaction strength can be controlled by Feshbach resonance. Optical trapping methods in combination with magnetic tuning of the scattering properties directly lead to the experimental achievements of Bose-Einstein condensation (BEC) of Cesium, which at first failed by using magnetic trapping approaches due to the large inelastic collision rate. The rapid loss of cesium atoms due to the inelastic two-body collisions greatly suppresses the efficient evaporative cooling to obtain a condensate. For optical production of cesium atomic BEC, it is necessary to prepare a large number of Cs atoms at specified state in an optical trap for condensation, especially for an efficient forced evaporation cooling. In this paper, we demonstrate our research on enhancing the loading rate of the atoms by using a dimple trap combined with a large-volume optical dipole trap (reservoir trap). In our work, the cold cesium atoms are prepared by a three-dimensional degenerated Raman sideband cooling, and then loaded into a large-volume crossed dipole trap by using the magnetic levitation technique. Effective load of the dimple optical trap is realized by superposing the small-volume dimple trap on the center of the largevolume optical trap. The theoretical analyses are performed for the magnetically levitated large-volume crossed dipole trap in variable magnetic field gradients and uniform bias fields. Optimal experimental values are acquired accordingly. The combined potential curve of the dimple trap, which is superimposed on the magnetically levitated large-volume dipole trap, is also given. The loading of precooled atoms from Raman sideband cooling into the magnetically levitated large-volume optical trap is measured in variable magnetic field gradients and uniform bias fields. Different loading results of the dimple trap are investigated, including direct loading after Raman sideband cooling, the large-volume optical trap and the magnetically levitated large-volume dipole trap without anti-trapping potential. Comparatively, the atomic number density is enhanced by a factor of ~15 by loading the atomic sample from the magnetically levitated large-volume dipole trap into the dimple optical trap. The experimental results lay a sound basis for the further cooling and densifying the atomic cloud through the evaporating cooling stage. This method can be used to obtain more cold atoms or a large number of Bose-Einstein condensation atoms for atomic species with large atom mass.
We report on an efficient production scheme for a large quantum degenerate sample of fermionic lithium. The approach is based on our previous work on narrow-line $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ laser cooling resulting in a high phase-space density of up to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This allows utilizing a large-volume crossed optical dipole trap with a total power of 45W45\phantom{\rule{0.16em}{0ex}}\mathrm{W}45W, leading to high loading efficiency and $8\ifmmode\times\else\texttimes\fi{}{10}^{6}$ trapped atoms. The same optical trapping configuration is used for rapid adiabatic transport over a distance of 25cm25\phantom{\rule{0.16em}{0ex}}\mathrm{cm}25cm in 0.9s0.9\phantom{\rule{0.16em}{0ex}}\mathrm{s}0.9s, and subsequent evaporative cooling. With optimized evaporation we achieve a degenerate Fermi gas with $1.7\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms at a temperature of 60nK60\phantom{\rule{0.28em}{0ex}}\mathrm{nK}60nK, corresponding to T/TF=0.16(2)T/{T}_{\text{F}}=0.16\left(2\right)T/TF​=0.16(2). Furthermore, the performance is demonstrated by evaporation near a broad Feshbach resonance creating a molecular Bose-Einstein condensate of $3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ lithium dimers.
We report the production of 87Rb Bose–Einstein condensate in an asymmetric crossed optical dipole trap (ACODT) without the need of an additional dimple laser. In our experiment, the ACODT is formed by two laser beams with different radii to achieve efficient capture and rapid evaporation of laser cooled atoms. Compared to the cooling procedure in a magnetic trap, the atoms are firstly laser cooled and then directly loaded into an ACODT without the pre-evaporative cooling process. In order to determine the optimal parameters for evaporation cooling, we optimize the power ratio of the two beams and the evaporation time to maximize the final atom number left in the ACODT. By loading about 6 × 105 laser cooled atoms in the ACODT, we obtain a pure Bose–Einstein condensate with about 1.4 × 104 atoms after 19 s evaporation. Additionally, we demonstrate that the fringe-type noises in optical density distributions can be reduced via principal component analysis, which correspondingly improves the reliability of temperature measurement.