Efficient Direct Evaporative Cooling in an Atom Chip Magnetic Trap

TL;DR

This paper demonstrates an efficient method for direct evaporative cooling of rubidium atoms on an atom chip, significantly increasing phase space density and enabling Bose-Einstein condensate production with minimal trap frequency changes.

Contribution

The study introduces a controlled trap depth reduction technique via chip currents and bias fields, achieving high phase space density increases and condensate formation.

Findings

Trap depth reduced by a factor of 20

Phase space density increased by over two orders of magnitude

Bose-Einstein condensates produced after minimal rf sweep

Abstract

We demonstrate direct evaporative cooling of $^{87}$Rb atoms confined in a dimple trap produced by an atom chip. By changing the two chip currents and two external bias fields, we show theoretically that the trap depth can be lowered in a controlled way with no change in the trap frequencies or the value of the field at the trap center. Experimentally, we maximized the decrease in trap depth by allowing some loosening of the trap. In total, we reduced the trap depth by a factor of 20. The geometric mean of the trap frequencies was reduced by less than a factor of 6. The measured phase space density in the final two stages increased by more than two orders of magnitude, and we estimate an increase of four orders of magnitude over the entire sequence. A subsequent rf evaporative sweep of only a few megahertz produced Bose-Einstein condensates. We also produce condensates in which raising the trap bottom pushes hotter atoms into an rf "knife" operating at a fixed frequency of 5\,MHz.