Coherent Rabi oscillations may also be driven by two-photon transitions. In this case we consider a system with three atomic energy levels, | 1 ⟩ ⟩ , | i ⟩ ⟩ , and | 2 ⟩ ⟩ , where | i ⟩ ⟩ represents a so-called intermediate state with corresponding frequency ω ω i , and an electromagnetic field with two frequency components: