Controlling the Stereodynamics of Cold Molecular Collisions

We report numerically-exact quantum scattering calculations for low-energy collisions of quantum-state prepared HD with H$_2$. Excellent agreement is obtained with recent measurements of Perreault et al. for the angular distribution of scattered HD at a collision energy of $\sim$ 1 K. By state-preparation of the HD molecules, control of the angular distribution of scattered HD was demonstrated. The stereo-dynamic control is achieved by the ability to choose a single or a coherent superposition of quantum states. We present a first-principles simulation of the experiment which enables us to attribute the main features of the observed angular distribution to a single $L=2$ partial-wave shape resonance.