Quantum theory of a neutral dipole in a radiation field

A physical interpretation is given for the Heisenberg equations implied by the usual phenomenological Hamiltonian for a neutral, magnetic dipole in a radiation field. If the Heisenberg equations that characterize the quantum description are to be formally the same as their classical counterparts, a distinction must be made between the canonical and orbital momenta, even for a neutral system. This leads to a novel interpretation of the spin-orbit energy and a reidentification of the electric polarizability of the system. The general ideas are illustrated by considering the exactly soluble special cases of a neutral magnetic dipole with a nonvanishing electric polarizability in uniform, static fields and in the field of a monochromatic, plane wave. A Lagrangian formalism corresponding to these ideas is also presented