Metastability and Quench Dynamics in a Long-Range Interacting Hubbard Model

Abstract

This thesis reports on the first experimental realization and study of a strongly correlated many-body system with tunable short- and global-range interactions using an ultracold atomic gas. Thereto, a Rubidium-87 Bose-Einstein condensate is loaded into a three-dimensional optical lattice, which strongly enhances on-site interactions between the atoms over their kinetic energy. While collisional interactions are naturally present in such a system, longer-ranged interactions are more elusive. By coupling the cloud to a single field mode of a high-finesse optical cavity, we achieve a global atom-atom coupling stemming from the delocalized nature of the cavity field. In a large range of parameters, this system is well described by a Bose-Hubbard model with additional global-range interaction. We map the phase diagram of the system by tuning the relative strengths of kinetic energy, short-range interactions, and global-range interactions, and detect four phases: a superfluid, a lattice supersolid, a Mott insulating, and a charge density wave phase. To study the system in the strongly interacting regime, we vary the strength of global-range interactions at the phase transition between the Mott insulator and the charge density wave at different rates and observe metastable behavior and hysteresis between the states. Our findings are supported by a theoretical toy model and indicate a first-order phase transition. While first-order phase transitions are generally common, they are unusual in ultracold atom experiments. From the photon flux leaking from the cavity, we deduce the microscopic dynamics of the transition. This dynamics points at an avalanche of several thousand atoms resonantly tunneling to their neighboring lattice sites within the timescale of the single particle dynamics. In a subsequent experiment, we study different types of global-range interactions acting on either the density or the spin of the system. Moving from scalar atom-light coupling between the cavity mode and the atomic cloud to a vectorial coupling, we observe the formation of a spin texture in a Bose-Einstein condensate of two internal spin states. One of the limiting factors for ultracold atom experiments is the long time needed to cool the atomic sample compared to the duration of the actual experiment. In the course of this thesis a range of technical upgrades of the experimental apparatus have successively reduced the preparation time by more than a factor of two. Additionally, the adoption of a comprehensive interlock framework allows for a continuous and remote operation of the machine by protecting the apparatus in case of malfunction.