Single-molecule–resolved imaging of
ultracold 87Rb133Cs molecules in an
optical lattice

Abstract

This thesis presents the construction of a spin resolved quantum gas microscope for ultracold molecules of 87Rb133Cs. Ultracold polar molecules in a lattice, with their long-range dipolar interactions, are a promising platform for quantum simulation of many-body systems quantum simulation. The ability to resolve single sites of the lattice, and measure the molecules rotational states opens up many interesting research possibilities. Such systems could be used to simulate a variety of many-body systems, investigating anisotropic spin models like the XXZ model, simulating the t-J model and realising exotic phases such as spin glasses and spin liquids. This work demonstrates the imaging of ultracold molecules in an optical lattice with single-molecule resolution, and spin-state resolution.

We prepare our molecules by associating high phase-space density (PSD) clouds of Rb and Cs. The clouds are prepared using a sequential loading scheme, where each species is loaded, cooled and evaporated separately, before the atom clouds are merged for association. In this thesis, we present and characterise the scheme used, whereby we can achieve the required high PSD mixture. This involves the use of a moving dimple trap and an evaporation sequence, which allows us to achieve a dual-species BEC. We then detail the association a cloud of weakly-bound Feshbach molecules and their transfer to the ground state using Stimulated Raman Adiabatic Passage (STIRAP). We discuss the characterisation of the Feshbach association process and the setup and optimisation of the STIRAP setup, including the laser system that locks to a high-finesse cavity, including the use optical feedforward to reduce the high frequency phase noise. This allows us to improve the STIRAP efficiency to 98.7(1)%, a record for RbCs.

We then introduce the microscopy setup used in the experiment, with the optical lattice and light sheet used to trap the atoms and molecules and the optical molasses used to image atoms of Rb and Cs while they are confined to a lattice site. We optimise and characterise the performance of this setup, for both Rb and Cs atoms, presenting a dual-species quantum gas microscope. We also discuss the methods used to identify single atoms in the lattice, and reconstruct the lattice using a neural-network to deconvolve the images. We finally load our molecular cloud into the optical lattice and dissociate the molecules into atoms, which act as a marker for each molecule, and are able to image the cloud with single-molecule resolution. We further leverage our ability to perform microscopy on both Rb and Cs to implement a sequence that maps the molecular rotational state to a specific species to be imaged, and realise a spin-resolved molecular microscope, where N=0 state is mapped to Rb, and the N=1 state to Cs.