Sympathetic Cooling with Two Atomic Species in an Optical Trap

We simultaneously trap ultracold lithium and cesium atoms in an optical dipole trap formed by the focus of a ${\mathrm{CO}}_2$ laser and study the exchange of thermal energy between the gases. The optically cooled cesium gas efficiently decreases the temperature of the lithium gas through sympathetic cooling. Equilibrium temperatures down to $25\mu \mathrm{K}$ have been reached. The measured cross section for thermalizing ${}^{133}\mathrm{Cs}{-}^7\mathrm{Li}$ collisions is $8\times {10}^{-12\mathrm{}}{\mathrm{cm}}^2$, for both species unpolarized in their lowest hyperfine ground state. Besides thermalization, we observe evaporation of lithium purely through elastic cesium-lithium collisions (sympathetic evaporation).