LOCALIZATION STABILIZED BY NOISE

We present a model which describes a quantum two-state system interact ing with the environment represented by stochastic noise. We show that coherent tunneling between the two states survives if the interaction with the environment is weak. On the contrary, a strong interaction d estroys quantum coherence and the system randomly jumps from one state to the other. Moreover, the jump probability rate becomes extremely s mall for very strong noise. The model is relevant for understanding th e quantum properties of some mesoscopic systems.