Ultracold and quantum degenerate mixtures of Cs and Yb

Abstract

The study of ultracold mixtures of neutral atoms builds on the techniques of trapping and cooling single species of atoms to access more complex physical systems, and provides a route towards formation of ultracold molecules. In this thesis, we experimentally produce and study ultracold mixtures of Cs and Yb. This choice of Cs and Yb gives a versatile experimental system, with high tuneability of reduced mass, intraspecies and interspecies scattering properties, and the ability to study Bose-Bose and Bose-Fermi atomic mixtures. A long-term goal of working with the mixture is production of ultracold CsYb molecules. The CsYb molecule has a 2-Sigma symmetry ground state, and both magnetic and electric dipole moments. An array of such CsYb molecules would therefore be a versatile experimental platform for applications such as quantum simulation of physical systems, quantum computation and ultracold quantum chemistry.

In this thesis, we report on experimental observations within the Cs-Yb mixture, made possible by the implementation of a bichromatic optical dipole trap consisting of individual dipole trapping beams of 532 nm and 1070 nm. This trap has been designed to overcome the problems of mismatched trap depths and in-trap losses previously encountered in a single-wavelength optical dipole trap. This allowed us for the first time to produce dual quantum degenerate mixtures. We report production of pure dual Bose-Einstein condensates (BECs) of Cs+174Yb with N_Cs~5x10^3 and N_Yb~7x10^4. We also report production of pure dual BECs of Cs+170Yb with N_Cs~1x10^4 and N_Yb~4x10^4. We further study these quantum degenerate mixtures, observing their lifetimes, dynamics, and properties such as mixture immiscibility and dual-species collapse.

We further improve the versatility of our experimental apparatus by implementing an optical lattice with tuneable wavelength. The lattice can operate at two Cs `tune-out' wavelengths in the region of 460 nm. This allows a species-specific lattice potential which traps only Yb. Alternatively, it can be operated at wavelengths where either the trapping potential or the trap frequency is balanced for Cs and Yb. We measure the polarizability of Cs with Kapitza-Dirac diffraction of a Cs BEC using the tuneable lattice. This helps us obtain a more precise measurement of the Cs tune-out wavelengths.

Finally, we investigate a potential route to formation of CsYb molecules by performing Feshbach spectroscopy on an ultracold mixture of Cs+173Yb. Using predictions supported by previous experimental and theoretical work, we report the first experimental observations of interspecies magnetic Cs-Yb Feshbach resonances. We observe two sets of resonances, at magnetic bias fields around 622 G and 702 G. These observations motivate further discussion on the utilisation of the resonances for magnetoassociation, as well as experimental techniques to detect and manipulate the nuclear spin substate composition of Fermionic Yb.