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 and momentum .
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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