  The evolution of the quantum wave packet describing an atom trapped in the
surface-tip junction of the scanning tunneling microscope is investigated by
using the time-dependent Schroedinger equation, and by a quasi-classical
Hamiltonian approach. The estimates concern a Xe atom in a biased double-well
junction potential. The exact treatment shows that quantum coherence
oscillations of the metastable ground state may occur at particular resonant
values of the bias voltage. The effect of decoherence by partial localization
is studied within the quasi-classical frame.

Quantum Physics