Dressed Ground State Research Articles

Long-time dynamics of self-dressing

We consider a system formed by a two-level atom initially in its bare ground state and the electromagnetic field in the absence of radiation. The interaction of the atom with the radiation field leads to the formation of a dressed ground state. To describe the dynamics of self-dressing, we study the time evolution of the permanence amplitude in the initial state. We adopt non-relativistic QED and take the atom-field interaction Hamiltonian in the minimal coupling form, within the electric dipole approximation. In order to follow the evolution of the permanence amplitude for long enough time periods we use the van Hove resolvent technique with appropriate partial summation of Feynman graphs. We show that the self-dressing process presents the typical behaviour of a decay process and occurs with a timescale a*-1, which is long compared with both the atom's inverse transition frequency 0-1 and the excited state decay time -1. This time is comparable with the lifetime of the excited state due to ordinary two-photon decay. We consider the reasons for such a long time period of self-dressing and make comparison with previous results.

Interference-induced optical gain without population inversion in cold, trapped atoms

Continuous-wave (cw) optical gain of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2} {\mathrm{cm}}^{\ensuremath{-}1}$ is obtained on a probe transition in a driven, three-level, V-type atomic system. The atoms exhibit no population inversion between the probe excited state and the dressed ground states of the combined atom-drive Hamiltonian. This gain without population inversion is interpreted as direct evidence of quantum interference, arising from coherences established in the atom by the applied optical fields. Agreement with a simple four-level theoretical model is excellent.

Causality and spatial correlations of the relativistic scalar field in the presence of a static source

The spatial correlations of the energy density of the virtual cloud around a static source of the relativistic bosonic scalar field are considered. An exact expression for the correlation function in the dressed ground state of the source is obtained which indicates antibunching of the virtual bosons. Subsequently the time-dependent undressing process is considered and superluminal changes of the spatial correlations are found. The problem of causality is discussed in detail in this framework.

Bare bound states and the quantum theory of measurement.

A bare bound state is modeled as a ground-state two-level atom which at t=0 is deprived of most of the low-frequency components of its virtual radiation field. This state is expected to evolve into a fully dressed ground state through reconstruction of the full cloud of virtual quanta. The quantum theory of measurement is used to show that the dynamics of the two-level atom due to this reconstruction can be detected experimentally under appropriate conditions. The measuring apparatus is modeled by a pointer coupled to the atom. The theory predicts that for sufficiently short measurements the bound state can be used as a probe of the dressing process. This result is discussed in connection with the theory of the excitonic polaron in a semiconductor.

Dressed and Half-Dressed Neutral Sources in Nonrelativistic QED

A nonrelativistic, quantum-mechanical neutral source is considered, which is coupled to the quantized e.m. field in vacuo. The dressed ground state of the coupled system is obtained in terms of the bare ground state and of other excited states in absence of coupling. This dressed ground state is characterized by a cloud of virtual photons in the space surrounding the source, which yields nonvanishing quantum averages of the magnetic and electric field energy densities. The source-field coupling is modeled by a Craig-Power Hamiltonian, and the ground state energy densities are shown to behave like r-7 at distance r from the dressed source. Half-dressed sources are introduced as sources surrounded by incomplete virtual photon clouds. The time evolution of an initially bare source is discussed in terms of time-dependent field energy densities in the Heisenberg picture and it is shown to lead asymptotically to a fully dressed ground state source. The results obtained, as well as the limits of validity of the approach adopted, are discussed in the light of previous theories for charged half-dressed sources in quantum electro- and mesodynamics.