Overlapping Bose-Einstein Condensates of Na and Cs

We report on the production of dual-species Bose-Einstein condensates of up to 1 x 10⁶ sodium atoms and 5 x 10⁴ cesium atoms, a key step towards the creation of ultracold ensembles of NaCs ground state molecules. NaCs molecules have a large dipole moment and are ideally suited to access strongly correlated states of dipolar quantum matter. We achieve dual condensation via sympathetic cooling of cesium by sodium in an optically-plugged magnetic trap and a crossed dipole trap. To associate molecules from dual condensates, the BECs must be overlapped, long-lived, and miscible. Using trapping light at 1064 nm, we find that the mass and polarizability ratios of Na and Cs almost perfectly compensate. As a result, the differential gravitational sag between the two species is negligible. With the scattering length of cesium tuned to a moderate value, we find that the condensates co-exist for several seconds. In addition, the condensates are miscible; using cross-thermalization, we determine that the interspecies scattering length between the Na |F, m_F>  = |1, 1> and Cs |3, 3> states is weakly repulsive. These conditions allow the association of NaCs Feshbach molecules from dual condensates.