Gravitational Sisyphus Cooling of87Rbin a Magnetic Trap

We describe a method for cooling magnetically trapped ${}^{87}\mathrm{Rb}$ atoms by irreversibly cycling the atoms between two trapped states. The cooling force is proportional to gravity. The atoms are cooled to 1.5 μK in the vertical dimension. We have extended this cooling method to two dimensions through anharmonic mixing, achieving a factor of 25 increase in the phase space density over an uncooled sample. This cooling method should be an important intermediate step toward achieving a Bose-Einstein condensate of Rb atoms.