Dynamics of the dissipative two-state system under ac modulation of bias and coupling energy

We consider a driven dissipative two-state system subject to periodic modulation of both of its two parameters. The exact master equation that governs the dynamics is derived. Closed form analytical solutions are presented within the noninteracting-blip approximation for the stochastic forces, and for the exactly solvable case alpha=1/2 of the Ohmic viscosity. We apply these results to a tight-binding particle with zero intrinsic bias. Selection rules are obeyed for arbitrary dissipation. For Ohmic dissipation, the effects of fast asymmetry modulation result in an overall reduction of quantum coherence and in a possible localization. On the contrary,a multiplicative modulation of the tunnelling coupling leads to an exponential enhancement of tunnelling.