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Physical Review A

Radiative emission due to atomic self-dressing in QED

G. Compagno, Davide Valenti

Published Feb 5, 2002• 5 citations
Publisher

Abstract

We study the radiative emission due to the self-dressing of a two-level atom, initially in its bare ground state, interacting with the zero-point electromagnetic field. Evolution in time leads to the formation of a dressed ground state of lower energy. This energy difference between bare and dressed ground state is taken into account by the emission of real photons. In order to describe this aspect of the self-dressing process we study the transition probability amplitude from the initial bare state to an asymptotic state consisting of the atom in its dressed ground state plus some real photons. Adopting nonperturbative techniques based on the resolvent method we find that the bare-dressed transition occurs at the lowest order with the emission of two photons. The spectral distribution obtained shows that there is a probability for emission of photons at frequency of the order of the atom's transition frequency.

DOI
10.1103/physreva.65.032106

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We study the radiative emission due to the self-dressing of a two-level atom, initially in its bare ground state, interacting with the zero-point electromagnetic field. Evolution in time leads to the formation of a dressed ground state of lower energy. This energy difference between bare and dressed ground state is taken into account by the emission of real photons. In order to describe this aspect of the self-dressing process we study the transition probability amplitude from the initial bare state to an asymptotic state consisting of the atom in its dressed ground state plus some real photons. Adopting nonperturbative techniques based on the resolvent method we find that the bare-dressed transition occurs at the lowest order with the emission of two photons. The spectral distribution obtained shows that there is a probability for emission of photons at frequency of the order of the atom's transition frequency.
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.
As shown by Hegerfeldt, positivity of energy in quantum mechanics leads to nonlocal effects, where a state that is initially localized immediately develops infinite tails. In a previous paper we have described the mechanism of such effects for relativistic free fields. In this paper we discuss the nonlocal effects for a more complex system that consists of a two-level atom interacting with a radiation field. We study the dressing process of the bare ground state in terms of the photon density. In this process there appears a nonlocal cloud that remains around the atom and ``Zeno'' photons that move away from the atom with the speed of light. Nonlocality is seen to be to due to a ``curtain'' effect where the cloud is initially covered by the Zeno photons, and immediately afterwards is uncovered as the photons move away from the atom. No superluminal speeds are involved. We also consider nonlocal effects associated with a partially dressed state where the atom is excited starting from the true ground state. The excitation of the atom produces a nonlocal rearrangement of the photon cloud, which leads to precursors to the usual retarded field.
We present a perturbative derivation of the fluorescence spectrum for a three-level atom driven by a classical radiation field. The energies of the three atomic levels are equal to \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathrm{i}}$ (i=1, 2, and 3) with ${\mathrm{\ensuremath{\omega}}}_{3}$\ensuremath{\gtrsim}${\mathrm{\ensuremath{\omega}}}_{2}$\ensuremath{\gtrsim}${\mathrm{\ensuremath{\omega}}}_{1}$ and ${\mathrm{\ensuremath{\omega}}}_{32}$\ensuremath{\gg}${\mathrm{\ensuremath{\omega}}}_{21}$ (${\mathrm{\ensuremath{\omega}}}_{\mathit{ij}}$=${\mathrm{\ensuremath{\omega}}}_{\mathit{i}}$-${\mathrm{\ensuremath{\omega}}}_{\mathit{j}}$). The radiation field, having a frequency \ensuremath{\Omega}\ensuremath{\gtrsim}${\mathrm{\ensuremath{\omega}}}_{31}$, can be considered to drive the 2-3 transition only. Spontaneous emission from the ground state, via a normally counter-rotating atom-vacuum interaction term in the Hamiltonian, conserves energy when a photon is subsequently absorbed from the laser field. The spectrum is explained in terms of a Raman process, and in terms of an interaction between the laser field and the coupled atom-vacuum field system. Connection is made with a dressed-atom description of the problem. \textcopyright{} 1996 The American Physical Society.
The interaction of a two-level atom uniformly moving along a classical trajectory with a high-Q cavity quantum mode is analyzed. The dressed-state method is used to derive a recurrence formula for the transition probability of the atom with photon emission; the temporal dynamics of this probability qualitatively depends on the Doppler shift of the atomic transition frequency, on the Rabi frequency of the atom-field system, and on the detuning of the atomic transition frequency from the field mode frequency. The emission dynamics of a moving atom is very sensitive to the detuning. Rabi-type oscillations with a frequency equal to the Doppler shift can arise under certain conditions. At resonance, the emission probability of a moving atom can considerably exceed the emission probability of an atom at rest. A plane-parallel-mirror cavity and a confocal spherical-mirror cavity are considered. It is shown that the peculiarities of Doppler-Rabi oscillations must be taken into account in micromaser theory.
An exact solution for the problem of two two-level atoms interacting with a single-mode detuned quantized radiation field is presented. On resonance, an asymptotic result is derived for large coherent initial fields. It is shown that when the atoms are initially prepared in one of the semiclassical eigenstates, the atom-plus-field wavefunction remains almost factored into an atomic and a field part throughout the interaction. Under certain conditions, the atomic part evolves into a unique pure state at half-revival time. The effects of the cavity detuning on the dynamics of the atomic level occupation probabilities and emission spectra are studied. It is found, for non-symmetrical excitation, that far off-resonance the field acts like a virtual level by means of which the energy is transferred between the atoms. Due to non-zero detuning, nine-peaked spectra are observed and are given a clear explanation from the dressed-state viewpoint. It is established that in the case of symmetrical excitation, the intensity of the emission spectrum is two times larger than that in the case of non-symmetrical excitation.
For a three-level atom in the ssV configuration (i.e., having two excited states each dipole-coupled to a common ground state), we have found a particular linear combination of bare-atom states in which Rabi oscillations and their associated collapses and revivals do not occur. Moving to a dressed-state picture, we discover that this particular linear combination state is just that dressed state which is decoupled from all the field modes. It is a dressed state for which the transition dipole moments with the other dressed states are zero. The existence of this decoupled dressed state depends on the tuning of the dressing laser field, which in turn depends on the bare-atom excited-state dipole moments and energy-level separation. When we include spontaneous emission, the population decays from the other dressed states into this decoupled state and remains coherently trapped there, producing a system that experiences no dynamical behavior. This is exact for \ensuremath{\delta}-function photon statistics (i.e., if there is no intensity uncertainty). The trapping becomes less perfect as the photon statistics are allowed to have a greater bandwidth. Also, if the applied field is tuned incorrectly, the spontaneous realignment of the atomic state amplitudes does not result in a totally decoupled dressed state, and the dynamics proceed normally.