Dressed States in Atoms and in Excitons

Abstract

Due to the coupling with the vacuum fluctuations of the electromagnetic field, a neutral ground-state atom is surrounded by a cloud of virtual photons, leading to the concept of dressed atom. A similar concept has been introduced in solid state physics in connection with the exciton-phonon coupling; this leads to the definition of the polaronic exciton, which can be pictorially represented as an exciton surrounded by a cloud of virtual phonons.

We briefly describe the structure of our model consisting of a two-level dressed atom and the physical nature of its virtual photon cloud. In the second phase of our work, we discuss the similarity of this model with a simplified model of the polaronic exciton in a semiconductor.

We review the quantum theory of measurement in connection with the population of dressed states. We discuss the role of the duration of the measurement with reference to the time scale provided by the vacuum fluctuations which contribute to the virtual cloud.

Finally we introduce the concept of partially dressed states as states whose virtual cloud is initially out of equilibrium and undergoes a regeneration process. We show that appropriate measurements on these states can be performed, in order to detect the time dependence of their population during the reconstruction of the virtual cloud. We discuss the formation of the virtual cloud of a newly created polaritonic exciton and we argue that this process could be experimentally detected. We stress the potentiality of experiments on the partially dressed polaronic exciton, which could provide models for fundamental. and as yet only partially explored, QED process such as positronium creation and annihilation.