Linear and nonlinear susceptibilities of a decoherent two-level system

The linear and nonlinear dynamical susceptibilities of a two-level system a re calculated as it undergoes a transition to a decoherent state. Analogous ly to the Glover-Tinkham-Ferrell sum rule of superconductivity, spectral we ight in the linear susceptibility is continuously transferred from a finite frequency resonance to nearly zero frequency, corresponding to a broken sy mmetry in the thermodynamic limit. In this respect, the behavior of the pre sent model (the Mermin model) differs from the spin-boson model. The third order nonlinear susceptibility, corresponding to two-photon absorption, has an unexpected nonmonotonic behavior as a function of the environmental cou pling, reaching a maximum within the decoherent phase of the model. Both li near and nonlinear susceptibilities may be expressed in a universal form.