Destruction of quantum coherence and wave packet dynamics

The development of short, powerful laser pulses and of sophisticated trapping techniques within the last few years has stimulated numerous theoretical and experimental investigations on the dynamics of wave packets in elementary, material quantum systems. These wave packets are non stationary, spatially localized quantum states which are situated on the border between the microscopic and macroscopic domain. A detailed understanding of their dynamics is essential for our conception of quantum mechanics and of its connection with classical mechanics. So far the interplay between classical and quantum mechanical aspects of their dynamics have been investigated in Rydberg systems (Alber and Zoller 1991), in molecules (Garraway and Suominen 1995, Sepulveda and Grossmann 1996), in clusters (Knospe and Schmidt 1996) and in nano structures (Koch et al. 1996). These studies have concentrated mainly on semiclassical aspects which may be attributed to the smallness of the relevant de Broglie wave lengths. Thereby quantum aspects still manifest themselves in interferences between probability amplitudes which are associated with various families of classical trajectories. However, for a comprehensive understanding of the emergence of classical behavior also a detailed understanding of the destruction of quantum coherence is required. Typically this destruction of coherence arises from external stochastic forces or environmental influences which cannot be suppressed. Though by now many aspects of the coherent dynamics of these wave packets are understood to a satisfactory degree still scarcely anything is known about the influence of destruction of quantum coherence. The main aim of this article is to discuss characteristic physical phenomena which govern the destruction of quantum coherence of material wave packets. For systematic investigations on this problem it is advantageous to deal with physical systems in which wave packets can be prepared and detected in a controlled way and in which the mechanisms causing the destruction of quantum coherence can be influenced to a large extent. Rydberg atoms (Seaton 1983, Fano and Rau 1986) are paradigms of elementary quantum systems which meet these requirements. The high level density of Rydberg states close to an ionization threshold is particularly convenient for the experimental preparation of spatially localized electronic wave packets by coherent superposition of energy eigenstates (Alber and Zoller 1991). Furthermore, the dynamics of electronic Rydberg wave packets exhibits universal features which apply to atomic and molecular Rydberg wave packets as well as to Rydberg wave packets in more complex systems such as clusters. This dynamical universality might be traced