New optical equations for the interaction of a two-level atom with a single mode of the electromagnetic field

Based on the theory of open quantum systems we derive new optical equations, more general than the conventional optical Bloch equations: new terms describing couplings of the observables through the environment and an asymmetry of the dephasing rates for the two polarization observables are obtained. We show that, due to the new coupling of the population with the polarization through the environment, negative values of the absorption coefficient are possible. We find experimental evidence of this new coupling by comparing the bistability characteristic obtained from the new equations, for a nonlinear Fabry-Perot resonator, with the experimental data of Sandle and Gallagher.