= Square-pulse resonance profile
{title2=$\left|\int_0^Te^{i\delta t}dt\right|^2=T^2\operatorname{sinc}^2(\delta T/2)$}
A constant-amplitude drive lasting $T$ gives the <Fourier transform> in the title, where $\operatorname{sinc}x=\sin x/x$ and its value at zero is one. The <transition probability> in weak driving has a finite resonant maximum, first zeros at $\delta=\pm2\pi/T$, and decreasing side lobes. Longer duration increases the peak as $T^2$ and narrows its width as $1/T$. The <rotating-wave approximation> discards a rapidly oscillating counterrotating term; it can have a small absolute error but a large relative error at the resonant term's zeros.
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