Stochastic resonance and nonlinear response in a dissipative quantum two-state system

We study the dynamics of a dissipative two-level system, driven by a monochromatic ac field, starting from the usual spin-boson Hamiltonian. The quantum Langevin equations for the spin variables are obtained. The amplitude of the coherent oscillations in the average position of the particle is studied in the high-temperature limit. The system exhibits quantum stochastic resonance in qualitative agreement with earlier numerical results.