Creating degenerate two-dimensional Bose gases with tunable interaction strength

Abstract

This thesis describes the experimental realisation of versatile and highly controllable quantum systems with ultracold quantum gases with the aim to study quantum many-body physics. In the first part we present the implementation of mesoscopic Fermi-Hubbard-type systems with potassium-40 in a bottom-up approach employing optical tweezers. The experimental setup features an in-vacuo microscope objective with a numerical aperture of 0.75 enabling high-resolution imaging, and allows for tunable interaction strengths via Feshbach resonances as well as for rapid repetition rates due to all-optical cooling schemes. However, reaching quantum degeneracy via Raman-sideband cooling turned out to be very challenging. The achieved 3D ground state fractions of ∼40% is not sufficient to make the study of quantum physics in the Hubbard model feasible. In the main part of the thesis we discuss how the experimental setup was re-designed and re-built in order to create two-dimensional bosonic bulk systems with potassium-39. The key feature is the rare combination of highly tunable interaction strengths via a variety of intra- as well as interstate Feshbach resonances with the ability to study spin mixtures with high-resolution imaging. We have been able to realise a three-dimensional quasi-pure Bose-Einstein condensate of ∼5 · 10^3 atoms. The sample is then brought into the quasi-two-dimensional regime by first pre-shaping it with a light sheet-like trap and then transferring it into a single layer of a blue-detuned optical lattice. As a result a degenerate two-dimensional Bose gas of ∼3 · 10^3 atoms with a temperature of ∼50nK corresponding to ∼0.2 T_{BKT} is created, where T_{BKT} is the BKT critical temperature. Finally, we discuss how 2D quantum droplets might be realised with the setup.