NONSPREADING ELECTRONIC WAVE-PACKETS AND CONDUCTANCE FLUCTUATIONS

We find strongly localized wave packets tracing without dispersion the classical dynamics of a Rydberg electron in a circularly polarized microwave field using a full 3D quantum calculation. These wave packets are single eigenstates of the atom dressed by the microwave field and are very stable against ionization. Their widths fluctuate with microwave amplitude and/or frequency realizing an optical analog of the conductance fluctuations in mesoscopic systems. Similar behavior is observed for linear microwave polarization. We show that such behavior is expected when classical nonlinear resonances are important in an atomic system.