Abstract
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.
Highlights
For a given beam size, we find that the optimal trap depths for loading and free evaporation differ
We show in figure 2 the potential VCDT in the z = 0 plane, truncated at three different energy levels
In the free evaporation step that follows, an increase in trap depth leads to a fast transfer of atoms from the arms to the central region, providing a dense sample
Summary
Our experiment starts with a sodium MOT capturing approximately 107 atoms in 10 s from a vapor whose pressure is modulated using light-induced atomic desorption [20]. During a first ‘dark MOT’ phase [21], we lower the power of the repumping laser in about 100 ms, from Irep = 300 to 10 μW cm−2 per beam while keeping the magnetic gradient on. This reduces the loss rate due to light-induced collisions by limiting the population of excited states [11]. We keep the cooling laser intensity at the same value as that for MOT loading, Icool = 0.9 mW cm−2 per beam, which corresponds to one sixth of the saturation intensity (Isat = 6.3 mW cm−2) During this ‘dark MOT’ phase, both the spatial density in the dipole trap and the temperature increase. The temperature of the atoms after this cooling sequence is around 50 μK
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