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arXiv:1104.3809

Causal signal transmission by quantum fields. Quantum electrodynamics in response representation

Lev I. Plimak, Stig Stenholm

Published Apr 19, 2011• 0 citations
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Abstract

Using electromagnetic interaction as an example, response transformations [L.P. and S.S., Ann.Phys. 323, 1963, 1989 (2008), 324, 600 (2009)] are applied to the standard perturbative approach of quantum field theory. This approach is rewritten in the form where the place of field propagators is taken by the retarded Green function of the field. Unlike in conventional quantum-field-theoretical techniques, the concept of space-time propagation of quantized field is built into our techniques.

DOI
10.48550/arxiv.1104.3809

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Using electromagnetic interaction as an example, response transformations [L.P. and S.S., Ann.Phys. 323, 1963, 1989 (2008), 324, 600 (2009)] are applied to the standard perturbative approach of quantum field theory. This approach is rewritten in the form where the place of field propagators is taken by the retarded Green function of the field. Unlike in conventional quantum-field-theoretical techniques, the concept of space-time propagation of quantized field is built into our techniques.
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Classically, electromagnetic pulses are described by real fields that couple to charged matter and propagate causally. We will show here that real fields of the form used in standard classical electromagnetic theory have a quantum mechanical interpretation in which the probability density for a photon to be at x\mathbf{x}x is positive definite and operators representing all of the standard physical observables exist. A covariant alternative to the ${\ensuremath{\omega}}_{k}^{1/2}$ dependence that appears in most (but not all) presentations of quantum optics and quantum field theory is presented and real Lorentz scalar one-photon advanced and retarded potentials are derived.
In the quantum theory of interacting electrons, a physically correct theory of time dependence should in principle be formulated as a relativistic quantum field theory. The physical model is that of electrons, each characterized by a probability distribution over space-time events x i , t i , that interact indirectly through the quantized electromagnetic field. This theory is simplified for particular applications by neglecting true radiative effects of quantum electrodynamics, and by passing to the limit of large c , the velocity of light in vacuo .
A field-quantized electromagnetic propagator description emphasizing the spatial localization of light emitted from a single-electron atom is presented. With the aim of obtaining a better insight into the quantum electrodynamics in the near-field zone of the atom, two approaches, formally different but equivalent in respect to their physical predictions, are studied. From the Maxwell equations, among the transverse operators, a transverse propagator picture in which the source region for the field is identified with the spatial domain occupied by the transverse current density of the atom emerges. In this picture the field propagation is isotropic and always retarded. By identifying the source region with that of the total atomic current, the transverse propagator becomes anisotropic, and attains a nonretarded self-field part and a near-field part different from zero only for spacelike events. By changing the Coulomb Lagrangian, a nonrelativistic Hamiltonian formalism adequate for reconciling the photon concept with the anisotropic propagator picture is introduced. In this formalism the transverse self-field energy is transferred to the particle Hamiltonian, and the interaction Hamiltonian now includes an interaction between the retarded field and the transverse self-field dynamics. The link between the standard (nonpropagator) theory, often used to argue that the total electromagnetic field is always retarded, and the transverse propagator theory is established. The equivalence between the two approaches is proved by demonstrating that the standard theory in fact includes a nonretarded response in the spatial domain occupied by the longitudinal part of the induced atomic current density.
The response of the system, consisting of two kinds of opposite-charged fermions and their bound states (hydrogen-like atoms), to the perturbation by the external electromagnetic field in low particle kinetic energies region is studied. Expressions for Green functions that describe the system response to the external electromagnetic field and take into account the presence of particle bound states (atoms) are found. Macroscopic parameters of the system, such as conductivity, permittivity and magnetic permeability in terms of these Green functions are introduced. As an example, the perturbation of the ideal hydrogen-like plasma by the external electromagnetic field in low temperature region is considered. Such approach also enables to study the propagation properties of the signal, tuned up to the transition between two hyperfine ground state levels of alkali atoms that are considered in Bose-Einstein condensation (BEC) state. It is shown that the signal can propagate in such system with rather small energy loss. Such fact allows to introduce the group velocity concept and to study the slowing down conditions for the microwave signal that propagates in BEC.