Quantum Degeneracy in an Atomic Fermi-Fermi-Bose Mixture

This thesis deals with dilute fermionic and bosonic quantum gases in the nK temperature regime. In the work presented here, the first quantum-degenerate mixture of two different fermionic atomic species and the first triple-degenerate Fermi-Fermi-Bose mixture were produced. The quantum-degenerate mixtures were realized using sympathetic cooling of the fermionic species 6Li and 40K by an evaporatively cooled gas of bosonic 87Rb atoms in a magnetic trap. The apparatus, which was designed and built from scratch in this thesis work, provides a very versatile platform for a broad range of experiments. As the first important step in the production of the quantum-degenerate three-species mixture, simultaneous magneto-optical trapping of 6Li, 40K, and 87Rb was achieved, thereby demonstrating the first three-species magneto-optical trap (“triple MOT”) and the first two-fermion MOT. The triple MOT is loaded with lithium from a Zeeman slower and with potassium and rubidium from the background vapor that is maintained by atomic vapor dispensers. For this purpose, dispensers for potassium enriched in the isotope 40K were produced. The triple MOT was characterized and studied with respect to light-assisted interspecies losses. Furthermore, a sequence combining a compressed MOT and a temporal dark MOT is presented that, in spite of the differing requirements of the three species, allows us to achieve favorable starting conditions for further cooling in the QUIC-type magnetic trap. A magnetic transport system is used to transfer the precooled atomic clouds from the MOT position into the QUIC trap. The transport scheme was adapted to the specific requirements of the three-species mixture, in particular to the temperature of laser-cooled lithium, which is high in comparison to other species. The quantum-degenerate regime is reached by evaporative and sympathetic cooling in the QUIC trap. Species-selective evaporation of rubidium is achieved by driving a 87Rb hyperfine transition to an untrapped state. In order to avoid undesired atom losses and heating due to spin-exchange collisions, the atoms are prepared in the maximally polarized internal ground states. Careful state cleaning during the cooling process was found crucial to attain quantum degeneracy. Since evaporative and sympathetic cooling both rely on energy transfer by elastic collisions between atoms, the efficiency of the cooling process depends on the relevant elastic scattering cross sections. The 87Rb-87Rb and 40K-87Rb scattering cross sections are quite favorable for this purpose. The 6Li-87Rb scattering cross section, however, was unknown at the beginning of the project and was later found to be comparatively small, making sympathetic cooling of 6Li by 87Rb rather challenging. In this thesis work, an experimental sequence was developed that nevertheless allows the production of a quantum-degenerate 6Li gas with large atom number in this mixture. Furthermore, it is shown that in the three-species mixture, the efficiency of the cooling process for 6Li is significantly increased by the presence of 40K through catalytic cooling. The quantum-degenerate Fermi-Fermi and Fermi-Fermi-Bose mixtures realized in this thesis work serve as a starting point for a broad range of possible future experiments, including the creation of heteronuclear Fermi-Fermi dimers and the investigation of the BEC-BCS cross-over regime. In this context, the possibilities opened by the mass difference and the differing internal structures of the two fermionic species are of particular interest.