Raman transition

A Raman transition changes an atom's internal state through a two-photon interaction involving an intermediate level. In a stimulated Raman transition, absorption from one applied beam and stimulated emission into another supply a net energy $\hslash ({\omega }_1-{\omega }_2)$ and momentum $\hslash ({\mathbf{k}}_1-{\mathbf{k}}_2)$.

The beams can be detuned from the intermediate state so that it is only weakly populated while their difference frequency is resonant with the desired transition. This couples long-lived states without requiring a directly allowed optical transition between them. Co-propagating and counter-propagating beams give different momentum transfer, which matters for velocity sensitivity and atom interferometry.

Coherent absorption from one applied beam and stimulated emission into another, transferring an atom between two long-lived states. The energy transfer uses the frequency difference and the momentum transfer the wavevector difference. It is distinct from an absorption-absorption process.

Eliminating a common far-detuned intermediate state with single-photon couplings $\hslash {\Omega }_1/2$ and $\hslash {\Omega }_2/2$ gives effective Rabi angular frequency of magnitude $|{\Omega }_1{\Omega }_2|/(2|\Delta |)$. The complex laser phases set the coupling phase, and differential AC Stark shifts change the two-photon resonance.