= Raman transition
{c}
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> $\hbar(\omega_1-\omega_2)$ and <momentum> $\hbar(\mathbf k_1-\mathbf k_2)$.
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>.
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