= Rotating-frame Rabi Hamiltonian
{title2=$H_R=(\Omega_0-\omega)S_z+\Omega_RS_x$}
A circularly driven <spin> becomes time-independent after a rotation $R=e^{-i\omega tS_z/\hbar}$. The transformed <Hamiltonian operator> is $R^\dagger HR-i\hbar R^\dagger\dot R$, so the longitudinal <frequency> is shifted by $-\omega$. This is an exact change of frame for a circular drive. The <rotating-wave approximation> is a separate approximation needed when a drive also contains a counterrotating component.
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