Quantum degenerate mixtures of strontium and rubidium atoms

We report on the realization of quantum degenerate gas mixtures of the alkaline-earth-metal element strontium with the alkali-metal element rubidium. A key ingredient of our scheme is sympathetic cooling of Rb by Sr atoms that are continuously laser cooled on a narrow-linewidth transition. This versatile technique allows us to produce ultracold gas mixtures with a phase-space density of up to 0.06 for both elements. By further evaporative cooling we create double Bose-Einstein condensates of ${}^{87}$Rb with either ${}^{88}$Sr or ${}^{84}$Sr, reaching more than $1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ condensed atoms per element for the ${}^{84}$Sr-${}^{87}$Rb mixture. These quantum gas mixtures constitute an important step towards the production of a quantum gas of polar, open-shell RbSr molecules.