Strongly Interacting Quantum Mixtures of Ultracold Atoms

This thesis describes the construction of a new apparatus for ultracold quantum gases as well as the scientific results this machine has produced so far. This new apparatus is capable of simultaneously cooling and trapping lithium, sodium, and potassium. It therefore provides a platform to study a large variety of quantum mixtures. Three main experimental results are presented. Firstly, the direct cooling of K to Bose-Einstein condensation is presented. Then the K atoms provide the coolant for Li and K, achieving a triply degenerate gas of Li -K -K. In particular, a broad interspecies Feshbach resonance between K -K is observed, opening a new pathway to study a strongly interacting isotopic Bose-Fermi mixture of K -K. Secondly, a new Bose-Fermi mixture of Na -K is introduced. We show that Na is a very efficient coolant for K by sympathetically cooling K to quantum degeneracy with the help of a Na condensate. Moreover, over thirty interspecies Feshbach resonances are identified, paving the way to study strongly interacting BoseFermi problems, in particular the Bose polaron problem. Thirdly, we report on the first formation of ultracold fermionic Feshbach molecules of NaK by radio-frequency association. The lifetime of the nearly degenerate molecular gas exceeds 100ms in the vicinity of the Feshbach resonance. The NaK molecule features chemical stability in its ground state in contrast to the case of the KRb molecule. Therefore, our work opens up the prospect of creating chemically stable, fermionic ground state molecules of NaK where strong, long-range dipolar interactions will set the dominant energy scale. Finally, the thesis concludes with an outlook on future topics in polaron physics and quantum dipolar gases, which can be studied using the new apparatus. Thesis Supervisor: Martin W. Zwierlein Title: Silverman Family Career Development Associate Professor of Physics