Heteronuclear quantum gas mixtures

Abstract

In this PhD tutorial, we present experiments with quantum degenerate mixtures of fermionic and bosonic atoms in three-dimensional optical lattices. This heteronuclear quantum gas mixture offers a wide range of possibilities for quantum simulation, implementation of condensed matter Hamiltonians, quantum chemistry and ultimately dense and quantum degenerate dipolar molecular samples. We show how quantum degenerate mixtures of 40K and 87Rb are created in the experiment. We analyse stages of evaporative cooling and show how a dynamic mean-field collapse occurs during the final stage of the evaporation (Ospelkaus et al 2006 Phys. Rev. Lett. 96 020401) as a result of attractive interactions. The particle numbers observed in our experiment have only been limited by this mean-field collapse, resulting in an excellent starting point for our experiments. We explore magnetic-field-induced Feshbach resonances and demonstrate tuning of interactions (Ospelkaus et al 2006 Phys. Rev. Lett. 97 120403) between 40K and 87Rb by means of heteronuclear Feshbach resonances. We observe both stable attractively and repulsively interacting mixtures. We analyse the mean-field energy of the condensate and find qualitative agreement with a simple model. By making the interaction strong and attractive, we induce a mean-field collapse of the mixture. For strong and repulsive interactions, we observe phase separation of the mixture. When loaded into a 3D optical lattice, a whole zoo of novel quantum phases has been predicted for Fermi–Bose mixtures. We present the first realization of Fermi–Bose mixtures in 3D optical lattices as a novel quantum many-body system (Ospelkaus et al 2006 Phys. Rev. Lett. 96 180403). We study the phase coherence of the bosonic cloud in the 3D optical lattice as a function of the amount of fermionic atoms simultaneously trapped in the lattice. We observe a loss of phase coherence at much lower lattice depth than for a pure bosonic cloud and discuss possible theoretical scenarios including adiabatic processes, mean-field Fermi–Bose–Hubbard scenarios and disorder-enhanced localization scenarios. After considering this many-body limit of mixtures in lattices, we show how fermionic heteronuclear Feshbach molecules can be created in the optical lattice (Ospelkaus et al 2006 Phys. Rev. Lett. 97 120402) as a crucial step towards all ground-state dense dipolar molecular samples. We develop rf association as a novel molecule association technique, measure the binding energy, lifetime and association efficiency of the molecules. We develop a simple theoretical single-channel model of the molecules trapped in the lattice (Deuretzbacher et al 2008 Phys. Rev. A 77 032726) which gives an excellent quantitative agreement with the experimental data.