Rydberg Atoms and the Test of Simple Quantum Electrodynamical Effects

Abstract

Rydberg Atoms represent an ideal testing ground for some of the most fundamental models and predictions of low-energy quantum electrodynamics (QED). The following are reasons and examples:

• (a) The matrix elements for electric dipole transitions between neighbouring Rydberg states scale as n², where n is the principal quantum number. For high enough n, stimulated effects overcome spontaneous emission already for very small photon numbers. As a consequence, Rydberg atoms are very sensitive e.g. to blackbody radiation (see Ref. [1] and [2] for recent reviews).

• (b) The transitions to neighbouring levels are in the region of millimeter waves, therefore it is possible to physically modify the nature of the environment into which they decay, using for example conducting walls. Introducing conductors imposes boundary conditions on the electromagnetic field, and leads back to a descrete spectrum in the case of a finite volume enclosed in a cavity. In principle, there are essentially two distinct cases to be discussed. First, the situation of an atom in close proximity to a conducting plate [3–8]. The induced image charges give rise to extra contributions of a van der Waals-type force to the inner atomic forces, thus leading to position-dependent level shifts. Second, there are effects from a discrete mode structure of the electromagnetic field inside a cavity. due to its geometry. Of course, it is not possible to consider one of these phenomena without the other, but in most cases only one of the two produces the major influence. Consequences of the discrete mode structure of a cavity for Rydberg atoms are: the rate of the spontaneous emission is enhanced or diminished,depending upon the cavity being tuned on or off resonance with a transition frequency [9–13], as well as modifying the Lamb shift of Rydberg levels [14].

• (c) For cavities with high quality factors, the photon emitted by an atom in a Rydberg state remains stored inside the resonator long enough to be reabsorbed by the same atom with a finite probability. In this way, it is possible to realize a single-atom maser [15]. A single Rydberg atom inside a low-loss, single-mode resonator is an experimental realization of the Jaynes-Cummings model [16], describing the interaction between a single two-level atom and a single mode of the electromagnetic field. This model has been the object of considerable attention in the past, and a number of purely quantum mechanical predictions on the dynamics of this system have been made. These include the collapses and revivals in the dynamics of the atomic population. Rydberg atoms will for the first time offer the possibility to test these predictions [16–18].