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arXiv:1409.4502 · Physical Review A

Optical runaway evaporation for the parallel production of multiple Bose-Einstein condensates

Amita B. Deb, Thomas McKellar, Niels Kjærgaard

Published Nov 17, 2014• 12 citations
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Abstract

We report on parallel production of Bose-Einstein condensates (BECs) in steerable, multiplexed crossed optical dipole traps. Using a conventional trap-weakening evaporation scheme, where the optical trapping power is lowered, we obtain an array of up to four independent BECs. To improve evaporation efficiency, we propose to target each crossed trap site with a narrow auxiliary laser beam, creating an escape channel for energetic atoms. We experimentally demonstrate runaway evaporation using this scheme, which is characterized by very modest weakening in atomic confinement such that high densities and elastic collision rates can be maintained. Based on discretely time-shared optical tweezers, our approach is particularly suited for addressing the problem of simultaneously cooling atoms in multiple traps, providing the freedom to act locally and in a tailored fashion at individual trap sites.

DOI
10.1103/physreva.90.051401

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We report on parallel production of Bose-Einstein condensates (BECs) in steerable, multiplexed crossed optical dipole traps. Using a conventional trap-weakening evaporation scheme, where the optical trapping power is lowered, we obtain an array of up to four independent BECs. To improve evaporation efficiency, we propose to target each crossed trap site with a narrow auxiliary laser beam, creating an escape channel for energetic atoms. We experimentally demonstrate runaway evaporation using this scheme, which is characterized by very modest weakening in atomic confinement such that high densities and elastic collision rates can be maintained. Based on discretely time-shared optical tweezers, our approach is particularly suited for addressing the problem of simultaneously cooling atoms in multiple traps, providing the freedom to act locally and in a tailored fashion at individual trap sites.
We demonstrate runaway evaporative cooling directly with a tightly confining optical-dipole trap and achieve fast production of condensates of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 87Rb^{87}\text{R}\text{b}87Rb atoms. Our scheme uses a misaligned crossed-beam far off-resonance optical-dipole trap (MACRO-FORT). It is characterized by independent control of the trap confinement and depth allowing forced all-optical evaporation in the runaway regime. Although our configuration is particularly well suited to the case of 87Rb^{87}\text{R}\text{b}87Rb atoms in a 1565 nm optical trap, where an efficient initial loading is possible, our scheme is general and will allow all-optical evaporative cooling at constant stiffness for every optically trappable atomic or even molecular species.
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.
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 demonstrate a simple scheme to achieve fast, accelerating (runaway) evaporative cooling of optically trapped atoms by tilting the optical potential with a magnetic field gradient. Runaway evaporation is possible in this trap geometry due to the weak dependence of vibration frequencies on trap depth, which preserves atomic density during the evaporation process. Using this scheme, we show that Bose-Einstein condensation with $\ensuremath{\sim}{10}^{5}$ cesium atoms can be realized in 2−−4s2--4\phantom{\rule{0.3em}{0ex}}\mathrm{s}2−−4s of forced evaporation. The evaporation speed and energetics are consistent with the three-dimensional evaporation picture, despite the fact that atoms can only leave the trap in the direction of tilt.
We report on an all-optical production of a 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate (BEC) of 106{10}^{6}106 atoms. We construct a double compressible crossed dipole trap (DCDT) formed by a high-power multimode fiber laser (MCDT) and a single-mode fiber amplifier (SCDT), which are both operated at $1.06\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. A very cold dense gas is first cooled by polarization gradient cooling in a three-dimensional optical lattice. More than $2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms are loaded into the enlarged DCDT. Both CDTs are then simultaneously compressed to significantly different sizes followed by evaporation, which is performed by lowering only the MCDT power. The tighter SCDT produces an extremely high collision rate and maintains the trap stiffness, which leads to rapid and efficient evaporation. After 0.4 s, a gas of $5\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms with a phase-space density of 0.2 is confined within the SCDT alone. Further evaporation in 2.8 s yields a nearly pure BEC of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms in the $|F{m}_{F}\ensuremath{\rangle}=|11\ensuremath{\rangle}$ state. This number is the largest generated among all-optical methods. Our approach significantly improves the atom number of a condensate and circumvents the severe atom loss previously reported for multimode fiber lasers.
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 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.