Born-Oppenheimer and the Geometry of Ray Space

Abstract

It is known that, within the Born-Oppenheimer approximation, the slow modes of the nuclear motion are altered by three effects that emerge from integrating out the fast modes of the electronic motion. The first is an effective scalar potential $V_{\mathrm dyn}$ coming from the eigenvalue of the electronic state, the second is an effective magnetic field coming from the Berry phase vector potential $A$. The third term is an additional potential $V_{\mathrm geom}$ originating in the geometry of ray space and the Fubini-Study metric. In this article, we illustrate these effects and their geometric origin in the context of a simple toy model of a slow neutron interacting with a strong, spatially varying magnetic field. Regarding the neutron spin as a fast degree of freedom, we work out the slow dynamics of the motion of the neutron. Our treatment is geometrical and brings out the effects originating in the K\"ahler geometry of ray space and the Fubini-Study metric. We then give examples of magnetic field configurations which isolate these three separate effects. Finally we apply these ideas to the trapping of cold atoms. Our main result is that the geometric electric potential $V_{\mathrm geom}$ dominates for smaller traps and can be used to confine cold atoms in static traps. This observation could result in better and smaller atomic clocks. This paper is dedicated to Michael Berry on his 80th Birthday.