Rotating-wave approximation

The rotating wave approximation is an approximation used in atom optics and magnetic resonance. In this approximation, terms in a Hamiltonian which oscillate rapidly are neglected. This is a valid approximation when the applied electromagnetic radiation is near resonance with an atomic resonance, and the intensity is low. Explicitly, terms in the Hamiltonians which oscillate with frequencies ${\displaystyle \omega _{l}+\omega _{0}}$, are neglected while terms which oscillate with frequencies ${\displaystyle \omega _{l}-\omega _{0}}$ are kept, where ${\displaystyle \omega _{l}}$ is the light frequency, and ${\displaystyle \omega _{0}}$ is a transition frequency.