We demonstrate sympathetic cooling between magnetically trapped ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ atoms. Up to $6\times {10}^6$ ${}^{85}\mathrm{Rb}$ atoms are cooled via elastic collisions with a reservoir of $8\times {10}^8$ ${}^{87}\mathrm{Rb}$ atoms. The temperature of the binary atomic ensemble is adjusted by using radio-frequency evaporation of ${}^{87}\mathrm{Rb}.$ Temperatures as low as $T=100\mathrm{}\hspace{0.5em}\mathrm{nK}$ have been realized in the binary ensemble. Through a cross-dimensional mixing approach we have been able to measure the interspecies ${}^{87}\mathrm{Rb}{-}^{85}\mathrm{Rb}$ scattering cross section $\sigma =1.52\mathrm{}\left(34\right)\mathrm{}\times {10}^{-15}\hspace{0.5em}{\mathrm{m}}^2.$

• Received 22 December 2000

DOI:https://doi.org/10.1103/PhysRevA.64.021402