Localization stabilized by noise

We present a model which describes a quantum two-state system interacting 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 destroys quantum coherence and the system randomly jumps from one state to the other. Moreover, the jump probability rate becomes extremely small for very strong noise. The model is relevant for understanding the quantum properties of some mesoscopic systems.