We report on the creation of dual-species Bose-Einstein condensates (BECs) of Na 23
atoms and Cs 133 atoms. We demonstrate sympathetic cooling of Cs with Na in a magnetic
quadrupole trap and a crossed optical dipole trap, leading to Na BECs with 8 (1)× 10 5
atoms and Cs BECs with 3.5 (1)× 10 4 atoms. Investigating cross thermalization and
lifetimes of the mixture, we find that the Na and Cs BECs are miscible and overlapping,
interacting with a moderate interspecies scattering length of 18 (4) a 0 at 23 G and 29 (4) a 0 …

Mixtures of ultracold atoms play a central role in the creation of complex many-body systems
with ultracold quantum gases. They have opened the door to studies of coupled superfluids
[1], polarons [2–5], Efimov physics [6–8], and impurity physics in optical lattices [9, 10]. In
addition, ultracold atomic mixtures have enabled the efficient preparation of quantum gases
via sympathetic cooling, which has been demonstrated for bosons [11, 12] and fermions [13,
14] and has become the stepping stone for foundational studies of ultracold Fermi gases …