Dimers, Trimers and their Superpositions in a Bose-Fermi Mixture

Abstract

This thesis describes experiments on few- and many-body bound states in a Bose-Fermi mixture of ultracold 23Na and 40K atoms. We examine the formation of dimers and trimers in a balanced, thermal mixture and their evolution into strongly interacting Bose polarons with hybridized dimer and trimer character when we instead immerse an impurity concentration of K into a dense quantum bath of Na. We report a novel direct observation of a heteronuclear halo trimer, consisting of two lighter Na atoms and one heavier K atom, alongside the familiar NaK Feshbach dimer, using radiofrequency (rf) spectroscopy. We find that in proximity to a Feshbach resonance, the trimer feature closely follows the dimer resonance across an order-of-magnitude variation in binding energy. We show that the measured binding energies are consistent with our theoretical model of the trimer as having the structure of a Feshbach dimer weakly bound to one additional boson. We then study the fate of impurities interacting with a bosonic quantum bath, the paradigmatic Bose polaron scenario. By preparing an initial attractive polaron state, we probe previously inaccessible, highly-correlated Bose polaron states, again on the repulsive side of the Feshbach resonance. Deep within the condensate, the rf spectra no longer exhibit discrete dimer and trimer features as before, instead dominated by a single broad feature. We attribute this to the impurity-boson coupling becoming stronger than the dimer-trimer energy splitting, leading to hybridization of dimer and trimer states and, consequently, an effective level repulsion consistent with the spectra we observe. This experiment demonstrates the remarkable interplay between polaron physics and bound-state formation in a quantum environment.