During the two years of the project we have designed, constructed and validated an experimental apparatus well adapted to these goals. It gives access to quantum degenerate mixtures of potassium, and is designed to work with the three isotopes (two bosons, 39K and 41K, and one fermion, 40K). Exploiting a combination of gray molasses sub-Doppler cooling and a hybrid magnetic/optical trap, it routinely produces large Bose-Einstein condensates of 41K that can be used to sympathetically cool the other two potassium isotopes. This approach has allowed us to observe for the first time dual Bose-Einstein condensation of 39K and 41K, and to demonstrate an alternative approach for cooling 39K to quantum degeneracy.
In a first series of experiments we have performed an extensive study of the scattering properties of potassium Bose-Bose mixtures. We have located 27 new Feshbach resonances, including 39K-41K mixtures and spin mixtures of 39K and 41K. In collaboration with M. Tomza (ICFO and University of Warsaw) and A. Simoni (University of Rennes), we are currently exploiting these results to further constrain the model potentials available for potassium scattering.
In parallel, we have carried out preparatory work for the implementation of a tunable optical lattice of non-trivial topology in our apparatus, developing the required phase stabilization schemes, and designed, characterized and validated a high numerical aperture imaging system. The latter has already been implemented in the experimental apparatus, and should allow for the generation of the disordered external potentials required to demonstrate topological protection.
Finally, in a series of experiments not planned in the original proposal we have studied spin mixtures of 39K with repulsive interspin interactions and intraspin attractive interactions. In this system, quantum fluctuations stabilize the system against collapse and lead to the formation of quantum droplets: macroscopic self-bound states with liquid-like properties. We have experimentally studied quantum droplets in a quasi-one dimensional geometry, where they compete with more conventional bright solitons. We have determined the droplet properties (density and spin composition) as a function of atom number and interaction strength, and observed a transition between dilute bright solitons and liquid-like droplets, in good agreement with a simple theoretical model.
