Tunable dipolar interactions and collisional shielding in a quantum gas of polar molecules

Ultracold polar molecules open new directions in quantum science owing to their complex internal structure and long-range dipolar interactions. Creation of a quantum-degenerate molecular gas, described in the  first part of this thesis, is an important step toward unlocking the rich potential of molecular systems. However, simultaneously controlling both elastic and inelastic molecular interactions has remained an outstanding experimental challenge. Strong dipolar interactions have generally led to even stronger molecular losses, preventing the creation of deeply-degenerate and strongly dipolar molecular gases. In this thesis, we demonstrate several methods for tuning molecular interactions with external electric  elds. First, by tightly confining the molecules into a two-dimensional (2D) geometry and applying a strong electric field, we induced repulsive dipolar interactions that suppressed molecular losses while enhancing the elastic collision rate, allowing for direct evaporation of the molecules to below the Fermi temperature in 2D. Second, at particular electric field strengths, we observed resonant collisional shielding of rotationally excited molecules. At these resonances, the molecular loss rate could be tuned over nearly three orders of magnitude, and could be suppressed by up to a factor of 10. Third, we leveraged our highly controllable electric field to address and select single 2D layers of molecules, which allowed the observation of chemical reactions induced by interlayer dipolar interactions. Finally, we studied the intralayer interactions between molecules placed in a coherent superposition of rotational states, a first step toward engineering the collective spin dynamics in this system with long-range interactions.