Quantum derivation of the use of classical electromagnetic potentials in   relativistic Coulomb excitation
  We prove that a relativistic Coulomb excitation calculation in which the classical electromagnetic field of the projectile is used to induce transitions between target states gives the same target transition amplitudes, to all orders of perturbation theory, as would a calculation in which the interaction between projectile and target is mediated by a quantized electromagnetic field. 
B.F. Bayman and F. Zardi