This is the effective or two-photon Rabi frequency. It is the product of the individual Rabi frequencies for the | 1 ⟩ ⟩ → → | i ⟩ ⟩ and | i ⟩ ⟩ → → | 2 ⟩ ⟩ transitions, divided by the detuning from the intermediate state | i ⟩ ⟩ .