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 shift—are 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.
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 shift—are 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.
The minimal-coupling Hamiltonian for a nonrelativistic hydrogenic atom interacting with a photon field in the \ensuremath{\alpha}-Lorentz gauge is canonically gauge transformed to one with four arbitrary degrees of gauge freedom. The theory, nonrelativistic in the charge dynamics, is wholly covariant in its treatment of the scalar, transverse, and longitudinal electromagnetic fields. One degree of gauge freedom of the transformed Hamiltonian comes from the arbitrary scalar parameter contained within the \ensuremath{\alpha}-Lorentz gauge, while the others come from arbitrary parameters governing the gauge transformation. A Jaynes-Cummings-type model is constructed by suitable restrictions to the gauge freedom, and found to display an admixture of transverse and scalar interactions but no longitudinal component. The model gives the Lamb shift, correct to the second order of the electronic charge consistent with relativistic quantum electrodynamics.
The Hamiltonian for non-relativistic quantum electrodynamics can be expressed in terms of the usual dynamical variables for the field and charged particles and as a functional of a c-number vector field p(x;x') which is essentially arbitrary. This corresponds to the freedom to use the vector potential of he field in an arbitrary gauge since every p(x;x') determines a gauge condition for the potential. We study the S -matrix for two different choices of p(x;x') and calculate the difference. This is done directly in low-orders of perturbation theory and suggests a conjecture for the general formula for the difference, valid in any order of perturbation theory; we verify the validity of this conjecture by elementary means. On the energy shell the S- matrices are identical, i.e. gauge-invariant.
A pair of nonidentical two-level atoms, separated by a fixed distance R, interact through photon exchange. The system is described by a state vector which is assumed to be a superposition of four "essential states": (1) the first atom is excited, the second one is in the ground state, and no photon is present, (2) the first atom is in its ground state, the second one is excited, and no photon is present, (3) both atoms are in their ground states and a photon is present, and (4) both atoms are excited and a photon is also present. The system is initially in state (1). The probabilities of each atom being excited are calculated for both the minimally-coupled interaction and the multipolar interaction in the electric dipole approximation. For the minimally-coupled interaction Hamiltonian, the second atom has a probability of being instantaneously excited, so the interaction is not retarded. For the multipolar interaction Hamiltonian, the second atom is not excited before the retardation time, which agrees with special relativity. For the minimally-coupled interaction the nonphysical result occurs because the unperturbed Hamiltonian is not the energy operator in the Coulomb gauge. For the multipolar Hamiltonian in the electric dipole approximation the unperturbed Hamiltonian is the energy operator. An active view of unitary transformations in nonrelativistic quantum electrodynamics is used to derive transformation laws for the potentials of the electromagnetic field and the static Coulomb potential. For a specific choice of unitary transformation the transformation laws for the potentials are used in the minimally-coupled second-quantized Hamiltonian to obtain the multipolar Hamiltonian, which is expressed in terms of the quantized electric and magnetic fields.
A novel Hamiltonian scheme for non–relativistic quantum electrodynamics in which the gauge arbitrariness of the field potential is kept explicit is used to study the gauge–dependence properties of various versions of perturbation theory when resonance and line–broadening effects are admitted. Time–dependent perturbation theory is shown to have severe gauge–dependence problems unless ad hoc modifications are made. A time–independent formulation of S–matrix theory is then studied. Far from resonance, the S–matrix is gauge invariant in all orders of perturbation theory due to a very precise cancellation of gauge–dependent terms which requires, among other things, complete sets of intermediate states; energy conservation also has a crucial role. However, an obvious separation of the S–matrix into a resonant (pole) and non–resonant background leads to incomplete cancellation of the gauge–dependent terms. The introduction of the Heitler damping matrix into an integral equation for the T–matrix leads to a gauge–invariant result. This provides the basis for a gauge invariant S–matrix theory of atoms and molecules interacting with electromagnetic radiation that encompasses resonance and damping effects.
We develop a regularised arbitrary-gauge formulation of nonrelativistic quantum electrodynamics and use it to compare Coulomb and multipolar descriptions with a Lorentzian form factor. We analyse the effect of regularisation in perturbation theory, including alternative partitions of the Hamiltonian into free and interaction parts, and the limits of the electric dipole approximation. The regularised multipolar gauge exhibits a cut-off-dependent trade-off between the strength of individual interaction terms and the localisation of material subsystems that suppresses direct inter-atomic interactions. We discuss the implications of the framework for short-range phenomena, including Dicke criticality.
In recent studies on ultrastrong coupling between matter and light in cavities, the significance of gauge choice when employing the widely used two-level approximation has been highlighted. Expanding upon these investigations, we extend the analysis to waveguide QED, where we demonstrate that truncations performed in the dipole gauge also yield accurate results. To illustrate this point, we consider the case of a dipole coupled to a cavity array. Various numerical and analytical techniques have been employed to investigate the low-energy dynamics of the system. Leveraging these theoretical tools, we argue that single-photon scattering is an ideal method for investigating gauge-related issues. Our findings reveal two effects in the scattering spectra, which cannot be reproduced in a truncated model using the Coulomb gauge. First, the primary resonance is modified due to a Lamb shift contribution. Second, we observe asymmetric transmission amplitudes surrounding this resonance, reflecting the asymmetry of the spectral density in this model. Additionally, we explore other features in the scattering spectra resulting from ultrastrong couplings, such as the emergence of Fano resonances and inelastic channels. Finally, we propose an experimental test of our ideas in the context of circuit QED.