Perturbation Theory for Open Two-Level Nonlinear Quantum Systems

Perturbation theory is an important tool in quantum mechanics. In this paper, we extend the traditional perturbation theory to open nonlinear two-level systems, treating decoherence parameter γ as a perturbation. By this virtue, we give a perturbative solution to the master equation, which describes a nonlinear open quantum system. The results show that for small decoherence rate γ the ratio of the nonlinear rate C to the tunneling coefficient V （i.e., r = O/V） determines the validity of the perturbation theory. For small ratio r, the perturbation theory is valid, otherwise it yields wrong results.