Evaporative Cooling of Large Numbers of Magnetically Trapped Rb Atoms

Wolfgang Ketterle, N. J. van Druten

Makoto Yamashita

The dynamics of evaporative cooling of magnetically trapped 87 Rb atoms is studied on the basis of the quantum kinetic theory of a Bose gas. The theoretical optimization of the cooling trajectory clarifies that the acceleration of evaporative cooling around the transition point of Bose-Einstein condensation is very effective against serious three-body recombination loss. The number of condensed atoms is expected to be largely enhanced by the optimization as compared with the typical value produced by the conventional evaporative cooling, where the frequency of the radio-frequency magnetic field is swept exponentially. Next, we precisely examine how the growth of a condensate depends on the experimental parameters of evaporative cooling in this inefficient exponential-sweep cooling. We find that, due to large nonlinear three-body loss, the number of condensed atoms finally becomes insensitive to the initial number of trapped atoms, the initial temperature, and the bias field of a magnetic trap. This result opens up the experimental possibility that the final number of condensed atoms can be well stabilized against the shot-to-shot fluctuations of experimental conditions in evaporative cooling.

Ph. W. Courteille, Dian-Jiun Han, R. H. Wynar +1 more

We report Bose-Einstein condensation of large numbers of 87Rb atoms in a magnetic time-averaged potential trap. Over 1.5 X 109 atoms, initially captured in the TOP trap, are evaporatively cooled down to temperatures near 100 nK. Condensates of up to 2 X 105 atoms at peak densities higher than 1014 cm-3 are observed.

O.J. Luiten, H. G. C. Werij, M. W. Reynolds +3 more

Pengjun Wang, Dezhi Xiong, Zhengkun Fu +1 more

We investigate sympathetic cooling fermions 40 K by evaporatively cooling bosonic 87 Rb atoms in a magnetic trap with microwave and radio frequency induced evaporations in detail. The mixture of bosonic and fermionic atoms is prepared in their polarized spin states | F = 9/2, m F = 9/2〉 for 40 K and | F = 2, m F = 2〉 for 87 Rb, which is trapped in Quadrupole—Ioffe—Configuration trap. Comparing microwave with radio frequency evaporatively cooling bosonic 87 Rb atoms with sympathetically cooling Fermi gas 40 K, we find that the presence of rubidium atoms in the |2, 1〉 Zeeman states, which are generated in the evaporative process, gives rise to a significant loss of 40 K due to inelastic collisions. Thus, the rubidium atoms populated in the |2, 1〉 Zeeman states should be removed in order to effectively perform sympathetically cooling 40 K with the evaporatively cooled 87 Rb atoms.

Robert J. Thompson, Kristian Helmerson, Aephraim M. Steinberg +3 more