Experiment group: Giovanni Cennini, Carsten Geckeler, Gunnar Ritt and Martin Weitz

We have created a Bose-Einstein condensate by direct evaporative cooling in a tightly focussed all-optical trap. The trapping potential for atoms is formed by two crossed CO₂-laser beams (35 μm beam waist). In our experiment, initially 6 × 10⁷ rubidium atoms (⁸⁷Rb) from a thermal alkali background vapour are collected in a magneto-optical trap (MOT). After a subsequent temporal dark MOT phase, the MOT beams and the magnetic field are switched off and 10⁶ atoms are left in the dipole trapping potential alone. The phase space density of the optically cooled atoms is near 1/300. The atomic density of several 10¹³ atoms/cm³ allows for a high collisional rate. Further cooling is achieved evaporatively by lowering the trapping potential, and after a 3.5 s cooling period Bose-Einstein condensation is observed with 10⁴ atoms in the vibrational ground state.

Animation showing the creation of a BEC.
All images are taken 15 ms after releasing the atoms from the dipole trap.

Absorption images of a thermal cloud (left), a mixture of thermal cloud and BEC (center) and an almost pure BEC (right).