Quantum Principles of Two-Level Atomic Spectroscopy

Abstract

Until very recently a single atom interacting with an electromagnetic field could only be a topic of theoretical consideration because experimentally only ensembles of atoms were accessible. Atomic spectra and the probabilities of transitions in atoms had been successfully studied in atomic ensembles for a long time. Quantum mechanics enables one to calculate both spectra and probabilities. However, quantum mechanics predicts that any transition happens at a random moment of time. In accordance with this prediction a single atom irradiated by light will make random jumps between its ground and excited electronic states. However, these jumps cannot be observed if we detect emission from the whole atomic ensemble, because a statistical average hides the random character of the processes. Therefore they cannot be observed in ensembles. This is only one example of new aspects which emerge when we move from atomic ensembles to a single atom. In this book we intend to discuss not only problems of probabilistic and spectral calculations, but also various random characteristics of atoms and molecules, the so-called quantum trajectories, which are typical in the spectroscopy of single atoms or molecules. In order to prepare for the discussion of quantum trajectories, we have to discuss the basic principles underlying the spectroscopy of a single two-level atom interacting with a transverse electromagnetic field.