Lateral interatomic dispersion forces

Abstract

Van der Waals forces between atoms and molecules are universally assumed to act along the line separating them. Inspired by recent works on e↵ects which can propel atoms parallel to a macroscopic surface via the Casimir-Polder force, we predict a lateral van der Waals force between two atoms, one of which is in an excited state with non-zero angular momentum and the other is isotropic and in its ground state. The resulting force acts in the same way as a planetary gear, in contrast to the rack-and-pinion motion predicted in works on the lateral Casimir-Polder force in the analogous case, for which the force predicted here is the microscopic origin. We illustrate the e↵ect by predicting the trajectories of an excited caesium in the vicinity of ground-state rubidium, finding behaviour qualitatively di↵erent to that if lateral forces are ignored.