Single-Atom Oscillators

Abstract

Modern methods of laser spectro­ scopy allow us to study single atoms or ions in an unperturbed environment. This has opened up interesting new ex­ periments, among them the detailed study of radiation-atom coupling. In the following, two experiments of this type are reviewed: the single-atom maser and the resonance fluorescence of a single stored ion. The simplest and most fundamental system for studying radiation-matter coupling is a single two-level atom in­ teracting with a single mode of an elec­ tromagnetic field in a cavity. It received a great deal of attention shortly after the maser was invented, but at that time, the problem was of purely academic interest as the matrix elements describing the radiation-atom interaction are so small, the field of a single photon is not suffi­ cient to lead to an atom-field evolution time shorter than other characteristic times of the system, such as the excited state lifetime, the time of flight of the atom through the cavity and the cavity mode damping time. It was therefore not possible to test experimentally the fun­ damental theories of radiation-matter interaction which predicted amongst other effects: (a) a modification of the spontaneous emission rate of a single atom in a reso­ nant cavity, (b) oscillatory energy exchange bet­ ween a single atom and the cavity mode, and (c) the disappearance and quantum revival of optical (Rabi) nutation induced in a single atom by a resonant field. The situation has changed drastically in the last few years with the introduc­ tion of frequency-tunable lasers that can excite large populations of highly excited atomic states, characterized by a high main quantum number n of the valence electron. These states are generally call­ ed Rydberg states since their energy levels can be described by the simple Rydberg formula. Such excited atoms are very suitable for observing quantum effects in radiation-atom coupling for three reasons. First, the states are very strongly coupled to the radiation field (the induced transition rates between neighbouring levels scale as n4); se­ cond, transitions are in the millimetre wave region, so that low-order mode cavities can be made large enough to allow rather long interaction times; final­ ly, Rydberg states have relatively long lifetimes with respect to spontaneous decay (for reviews see Refs. 1 and 2). The strong coupling of Rydberg states to radiation resonant with transitions to neighbouring levels can be understood in terms of the correspondence princi­ ple: with increasing n the classical evolution frequency of the highly excited electron becomes identical with the transition frequency to the neighbouring level; the atom therefore corresponds to a large dipole oscillating with the reso­ nance frequency. (The dipole moment is very large since the atomic radius scales with n2).