Sudden vector-potential pulse
= Sudden vector-potential pulse
In <temporal gauge>, a spatially uniform step $\mathbf A(t)=\mathbf A_0\Theta(t)$ gives $\mathbf E=-\mathbf A_0\delta(t)$. A charged scalar Fourier mode retains its canonical <wavevector> while its <kinetic momentum> changes from $\hbar\mathbf k$ to $\hbar\mathbf k-e\mathbf A_0$. The resulting <frequency> jump mixes positive- and negative-frequency modes.