Leveraging Yb clock states to form lattices of ²S CsYb molecules

Ultracold molecules have emerged as an exciting and powerful platform for experiments in quantum science. The molecules have a rich internal structure of long-lived rotational, vibrational and spin states, and there are controllable long-range dipole-dipole interactions between them. This has motivated many proposed applications spanning ultracold chemistry, precision measurement, quantum computing and quantum simulation of many-body phenomena. The field is flourishing: recent highlights include the control of ultracold molecular collisions, the creation of quantum-degenerate Bose and Fermi gases, and the first entangling gates between molecules. Looking to the future, there is a need to bring new types of molecules into the ultracold regime. Currently, two types of ultracold polar molecules are prevalent: bialkali molecules, such as KRb and RbCs, that can be assembled from ultracold atoms, and free radicals, such as CaF and SrF, with properties that make direct laser cooling feasible. Bialkali molecules are produced at high densities, leveraging the powerful cooling of atoms. They do not possess an electronic magnetic moment. In contrast, laser-cooled molecules possess both an electronic magnetic moment and an electric dipole moment, but are produced at low densities. For many applications it is desirable to prepare the molecules in an optical lattice with a high fraction of the sites containing a single molecule. This has been achieved for bialkali molecules by preparing atom pairs in the lattice prior to forming the molecules. However, for simulating lattice spin models relevant to quantum magnetism, it is desirable to use molecules with both magnetic and electric dipoles. The low densities of laser-cooled molecules currently preclude this. We will address this challenge by extending the association technique that has proved so successful for bialkali molecules to a new class of molecules. Specifically, we will associate alkali-metal atoms (Cs) and alkaline-earth-like atoms (Yb) prepared in an optical lattice to form CsYb molecules possessing both magnetic and electric dipoles. Previous attempts to produce molecules of this type from ground-state atoms have been thwarted because the scattering resonances needed for the association are extremely narrow. However, our preparatory work has identified resonances involving the metastable clock states of Yb that are suitable for molecule formation. To find and successfully exploit these resonances for molecule formation requires a research programme integrating theory and experiment at all stages. Our approach blends established techniques from the quantum-gas, optical-lattice-clock and ultracold-molecule communities to: (1) Prepare Cs-Yb atom pairs in optical lattices. (2) Access scattering resonances using the metastable states in Yb and form molecules using magnetoassociation. (3) Stabilise the molecules by transferring them to the ground electronic state and then using stimulated Raman adiabatic passage to the lowest rovibrational energy level. The combination of Cs and Yb is particularly advantageous. Yb offers 6 different isotopes (bosons and fermions) with usable natural abundances. When combined with Cs this allows significant tuning of the reduced mass and hence the interspecies scattering properties. A priori, we do not know which isotope will be the most favourable and will therefore explore several, developing and refining the underpinning theory as we progress. Our success will usher in a new era for quantum simulation using ultracold molecules and our application of optical-clock technology to a molecular system will opening exciting new avenues for probing fundamental physics.