Fig. 1. Scheme outlining the optical Stark effect.

Coupling of a two-level system with a resonant photon field splits the degenerate states $∣0⟩∣n⟩$ and $∣\mathrm{X}⟩∣n-1⟩$ by the Rabi frequency Ω₀. For non-resonant optical fields, the effect is reduced to a mere shift of the states $∣0⟩∣n⟩$ and $∣\mathrm{X}⟩∣n-1⟩$. In the case of negative detuning, this results in an increase ΔE of the transition energy between the two-level ground state $∣0⟩$ and the two-level excited state $∣\mathrm{X}⟩$; a shift known as the optical Stark effect