2023 Theses Doctoral

Collisional Studies of Ultracold NaCs Molecules

Warner, Claire Lauren

Bulk samples of ultracold dipolar molecules present a promising platform for the studies of quantum many-body and few-body physics. Modern experiments with a high degree of experimental control over the molecular quantum states and interactions are particularly suitable for emulating systems of strongly-correlated matter, as well as for investigations of open questions in quantum chemistry. In the work presented in this thesis, we establish sodium-cesium (NaCs) molecules as a platform for the pursuit of such studies. Thanks to their large do+ipole moments, NaCs molecules are ideal for both investigations of interacting many-body systems and few-body physics.

In this thesis, I outline the design and realization of an experimental apparatus for generating ultracold ensembles of ground state NaCs molecules. We pursue the strategy of assembling molecules from ultracold atoms, so we begin by developing a scheme for cooling Na and Cs in tandem. We achieve the first simultaneous Bose-Einstein condensation of Na and Cs. Investigating cross-thermalization and lifetimes of the mixture, we find that the Na and Cs condensates are miscible and overlapping, and can coexist for tens of seconds. Overlapping condensates of Na and Cs offer new possibilities for many-body physics with ultracold bosonic mixtures and constitute an ideal starting point for the creation of ultracold ensembles of NaCs Feshbach molecules.

Optimizing the coherent transfer of loosely-bound Feshbach molecules to the ro-vibrational ground state requires a careful spectroscopic study of the excited state structure of NaCs. Investigating the excited rotational substructure of NaCs, we identify the highly mixed 𝑐³Σ⁺₁ ⎮v = 22> ∼ 𝑏³ 𝚷₁ ⎮v = 54 state as an efficient bridge for stimulated Raman adiabatic passage (STIRAP). We demonstrate hyperfine state-selective transfer into the NaCs ground state with an efficiency of up to 88(4)\%. Highly efficient transfer is critical for generating large samples of ground state NaCs molecules and achieving high-fidelity molecule detection.

Finally, we study the collisional properties of ground state NaCs molecules. The motivation behind this study is to understand the loss mechanism that occurs when two molecules collide and form a collisional complex. The mechanism is poorly understood, and experiments across different molecular species have yielded varying results. Given the many open questions regarding complex formation and lifetimes, the investigation of new molecular species such as NaCs is of particular importance. With the ability to study samples of NaCs molecules in the absence of trap light for extended periods of time, our data excludes (NaCs)₂ complex lifetimes below 135~ms, significantly longer than the existing theoretical prediction. Our investigation sheds light on the scattering properties of NaCs molecules and the validity of the theoretical framework typically used to characterize the (NaCs)₂ collisional complex.