text: Coherent Rabi oscillations may also be driven by two-photon transitions. In this case we consider a system with three atomic energy levels, , , and , where  represents a so-called intermediate state with corresponding frequency , and an electromagnetic field with two frequency components:
  Now,  may be much greater than both  and , or , as illustrated in the figure on the right. 
  A two-photon transition is not the same as excitation from the ground to intermediate state, and then out of the intermediate state to the excited state. Instead, the atom absorbs two photons simultaneously and is promoted directly between the initial and final states. There are two necessary conditions for this two-photon process (also known as a Raman process), to be the dominant model of the light-matter interaction:
  In words, the sum of the frequencies of the two photons must be on resonance with the transition between the initial and final states, and the individual frequencies of the photons must be detuned from the intermediate state to initial and final state transitions. If the latter condition is not met and , the dominant process will be one governed by rate equations in which the intermediate state is populated and stimulated and Spontaneous emission events from that state prevent the possibility of driving coherent oscillations between the initial and final states.
  We may derive the two-photon Rabi frequency by returning to the equations
  which now describe excitation between the ground and intermediate states. We know we have the solution
  where  is the generalized Rabi frequency for the transition from the initial to intermediate state. Similarly for the intermediate to final state transition we have the equations
  Now we plug  into the above equation for 
  Such that, upon solving this equation, we find the coefficient to be proportional to:
  This is the effective or two-photon Rabi frequency. It is the product of the individual Rabi frequencies for the  and  transitions, divided by the detuning from the intermediate state .