where Ω Ω n = 4 g 2 ( n + 1 ) + δ δ 2 is identified as the Rabi frequency. For the case that there is no electric field in the cavity, that is, the photon number n is zero, the Rabi frequency becomes Ω Ω 0 = 4 g 2 + δ δ 2 . Then, the probability that the two level system goes from its ground state to its excited state as a function of time t is