= Resonant quantum control
A near-resonant drive selects a transition and, in an appropriate rotating frame and <rotating-wave approximation>, controls a two-level effective <Quantum Hamiltonian>. If $H_{\mathrm{eff}}=\Omega(t)\sigma_x/2$ with fixed axis, then $U(T)=\exp[-i\sigma_x\int_0^T\Omega(t)dt/2]$. Pulse area fixes the rotation angle; drive phase chooses a transverse axis. Selectivity and neglect of rapidly oscillating terms require the appropriate detuning and drive-strength regime.
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