Rapid production of a Rb85 Bose-Einstein condensate in a double compressible optical dipole trap

Abstract

We develop an all-optical method to rapidly produce a large $^{85}\mathrm{Rb}$ Bose-Einstein condensate. $^{85}\mathrm{Rb}$ has a reputation as a difficult species to cool to quantum degeneracy due to its collisional properties. We solve these intrinsic difficulties by a combination of polarization gradient cooling in a 3D optical lattice and a compressible double-crossed dipole trap (DCDT). This lattice cooling circumvents the low rethermalization rate and loads more than ${10}^{7}$ atoms with a phase-space density larger than ${10}^{\ensuremath{-}3}$ into the enlarged DCDT. The subsequent compressions and evaporations in the size-adjustable DCDT can maximize the cooling efficiency while reducing unnecessary inelastic losses. We fix the bias magnetic field to 161 G where the $s$-wave scattering length is positive and large. The forced evaporations function well against inelastic collisions at an average density of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. A nearly pure condensate of $2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms is produced in 3.2 s of evaporation time.