From strongly-interacting Bose-Fermi mixtures to ultracold molecules

Abstract

This thesis describes experiments on ultracold quantum gases. First, I discuss quantum simulation involving mixtures of bosonic and fermionic atoms. Second, I present work on creating and controlling ultracold dipolar molecules of ²³Na⁴⁰K. The rich phase diagram of Bose-Fermi mixtures was studied with our system of bosonic ²³Na and fermionic ⁴⁰K atoms. When the fermions were immersed as a minority species within a Bose-Einstein condensate, the system realized the canonical Bose polaron quasiparticle, which is an important paradigm in condensed matter physics. We investigated the strongly-coupled Bose polaron as it approached the quantum critical regime of the Bose-Fermi mixture. Using radiofrequency spectroscopy, we probed the binding energy and decay rate as a function of temperature.

In particular, the decay rate was found to scale linearly with temperature near the Planckian rate k[subscript B]T/h⁻ in the unitarity-limited regime, a hallmark of quantum critical behavior. Bose-Fermi mixtures host a complex spectrum of collective excitations, which can shed light on their properties such as collisional relaxation rates, equilibrium equations of state, and kinetic coefficients. We probed the low-lying collective modes of a Bose-Fermi mixture across different interaction strengths and temperatures. The spin-polarized fermions were observed to transition from ballistic to hydrodynamic flow induced by interactions with the bosonic excitations. Our measurements establish Bose-Fermi mixtures as a fruitful arena to understand hydrodynamics of fermions, with important connections to electron hydrodynamics in strongly-correlated 2D materials. The second part of this thesis describes the creation and manipulation of ultracold molecules in their ground state.

Molecules have more tunable degrees of freedom compared to atoms, paving the way for studies of quantum state-controlled chemistry, quantum information, and exotic phases of matter. We created loosely-bound Feshbach molecules from ultracold atoms, then transferred those molecules to their absolute electronic, vibrational, rotational, and hyperfine ground state by stimulated Raman adiabatic passage. The rotational level structure, sample lifetimes, and coherence properties were studied, culminating in a demonstration of second-scale nuclear spin coherence times in an ensemble of NaK. Controlling the intermolecular interactions - which can be tunable, anisotropic, and long range - is an outstanding challenge for our field. We induced strong dipolar interactions via the technique of microwave dressing, an alternative to using static electric fields to polarize the molecules.

The origin of these dipolar collisions was the resonant alignment of the approaching molecules' dipoles along their intermolecular axis, resulting in strong attraction. Our observations were explained by a conceptually simple two-state picture based on the Condon approximation.