Incoherent limit of the Bloch equations: transition to a balance equation and the rate of a photoinduced process

This paper is mainly methodological in nature. We consider the correspondence between two widely used models when describing the interaction of resonant radiation with matter: the equations for the optical Bloch vector and a balance equation for the population of a two level system. In addition, when deriving a balance equation, we show how an expression for the probability per unit time (rate) of a photoinduced process (photoabsorption and stimulated emission) is naturally obtained.