Abstract
It has been proved by the semi-classical gauge invariant formulation (GIF) that the correct interaction operator for coupling the field-free material states with the radiation field must be the position form regardless of the gauge chosen for expressing the electromagnetic potentials, in accordance with the well-established principle of gauge invariance. The semi-classical GIF is now extended to the quantized radiation field interacting with matter by defining the energy operator for the quantized radiation field in the presence of matter. It will be shown in this paper that the use of the energy operator guarantees the position form of the interaction operator even in the Coulomb gauge, contrary to the conventional approach in which the dark material Hamiltonian is used to get the interaction operator of the momentum form. The multipolar Hamiltonian is examined in the context of the quantum mechanical gauge transformation.