GAUGE INVARIANT QED WITH ARBITRARY MIXING OF P.A AND Q.E INTERACTIONS

The minimal-coupling Hamiltonian for a non-relativistic bound system of two charges in the dipole approximation is gauge transformed to a Hamiltonian of a variable gauge which mixes the usual p·A and q·E interactions, and re-scales the charges. Various photon transition processes-single photon absorption and emission; two-photon scattering and absorption; the Lamb shiftare investigated by perturbation theory and found to be gauge independent, providing the transitions occur on the energy shell. It is demonstrated how a particular choice of gauge removes the counter-rotating terms and diagonalises the Hamiltonian of a coupled oscillator and two-level atom. © 1990 Taylor & Francis Ltd.