Towards faster Production of Strongly Interacting Bose Gases. Optical Gray Molasses Cooling of Potassium-39

Abstract or Introduction

The work described in this thesis can be divided into two parts. The first part presents studies of an ultracold 39K Bose gases in the unitary regime. The second part is concerned with the fast production of sub-Doppler 39K gases using optical Gray Molasses. At a magnetic Feshbach resonance, where inter-particle interactions become maximal and an ultracold gas enters the unitary regime, we use Ramsey interferometry to show that in a Bose gas three-body correlations arising from Efimov physics cannot be neglected.

We present a direct measurement of the contact densities C2 and C3, which quantify the strength of two- and three-body correlations, respectively, and serve as an important link between the micro- and macroscopic description of quantum many-body systems. Independently, a study of the hydrodynamic behaviour of a unitary thermal gas is presented. Using radio-frequency Rabi pulses, we rapidly quench interactions in a thermal 39K cloud expanding from an anisotropic trap. We observe aspect-ratio inversions mediated by collisions, and find intriguing deviations from theoretical expectations for high phase
space densities at unitarity.

Thereafter, the design, implementation and characterisation of a setup for fast cooling of 39K via Gray Molasses on the D1 line is presented. After a careful optimisation, this setup allowed us to cool ~ 2*10^10 potassium atoms from 350 µK down to 8 µK,
far below the Doppler limit, within ~ 10 ms. We analyse the dependence of the Gray-Molasses cooling on laser frequency detunings, test the impact of the light intensity and study the dynamics of the cooling process. Finally, the current state of work is sketched, including steps of optical pumping, magnetic transport of the cloud, and transfer into an optical dipole trap.