Driving-induced symmetry breaking in the spin-boson system

A symmetric dissipative two-state system is asymptotically completely deloc alized independent of the initial state. We show that driving-induced local ization at long times can take place when both the bias and tunneling coupl ing energy are harmonically modulated. Dynamical symmetry breaking on avera ge occurs when the driving frequencies an odd multiples of some reference f requency. This effect is universal, as it is independent of the dissipative mechanism. Possible candidates for an experimental observation are flux tu nneling in the variable barrier rf SQUID and magnetization tunneling in mag netic molecular clusters.