Cold and ultracold molecules have many potential applications. Three “grand challenges” that have been laid down for the field are quantum simulation, precision measurements and controlled chemistry.
Sympathetic cooling, in which molecules are cooled by contact with ultracold atoms, will be crucial in producing the low temperatures and high densities required to meet these challenges. To achieve it, it is essential to understand and control molecular collisions. The molecules are invariably held in traps formed with electric, magnetic or optical fields. Inelastic or reactive collisions cause trap loss, and must be prevented when loading optical lattices for applications such as quantum simulation. Collisions also cause uncertainties in precision measurements.
Understanding collisions between ultracold atoms has been essential to the whole field of ultracold atom physics. It will be even more important for ultracold molecules, where there are many more possible outcomes and much more scope for control by external fields and/or state selection. This is the domain of cold and ultracold chemistry.
At low temperatures, collisions occur over a small range of energies and a small number of angular-momentum states. As the temperature approaches zero, we can limit them to a single hyperfine state and a single partial wave. In the mK regime, collision energies become comparable to energy-level shifts caused by externally applied electromagnetic fields. The collision dynamics are very sensitive to reactant quantum state, even at the hyperfine level, and applied electromagnetic fields can lead to dramatic changes in reaction rates and branching ratios.
Our vision is to understand the dynamics of cold molecular collisions and build a toolkit to control their outcome. To control molecular collisions effectively, we need fine control over the internal state and translational motion of the reactants.
We will use magnetic fields and quantum-state selection at the hyperfine level, as well as state-of-the-art theory, to understand fundamental collision processes and mechanisms of control. We will apply this understanding to control whether collisions lead to reaction or to inelastic energy transfer. By suppressing reactive and inelastic collisions, we aim to achieve sympathetic cooling.
The new and improved understanding achieved through this project can be used to control collisions in more complex systems. Species of interest include triatomic molecules and polyatomic molecules with large hydrocarbon side-chains. With further developments in experiment and theory, the complexity can grow until the toolbox of control protocols is full.