= Circularly rotating magnetic drive
{title2=$\Omega=|\gamma|B_1$}
For positive <gyromagnetic ratio> in static field $B\hat z$, transverse components $B_1\cos(\omega t+\varphi)$ and $-B_1\sin(\omega t+\varphi)$ rotate with the <spin> precession. Their rotating-frame <Hamiltonian operator> is time independent, with transverse <Rabi angular frequency> $\Omega=\gamma B_1$. A linearly polarized field of peak amplitude $B_{\rm peak}$ contains two oppositely rotating components and gives resonant $\Omega=|\gamma|B_{\rm peak}/2$ in the <rotating-wave approximation>. Negative <gyromagnetic ratio> requires opposite rotation sense.
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