= Flux-threaded quantum rotor
{title2=$E_n=\hbar^2(n-f)^2/(2I),\quad f=e\Phi_B/(hc)$}
For the Gaussian-unit flux term $e\Phi_B\dot\phi/(2\pi c)$, <canonical momentum> is shifted by $e\Phi_B/(2\pi c)$. Periodic wavefunctions then give the displayed spectrum for integer $n$. The flux period is $hc/|e|$ in these units; the corresponding SI expressions omit $c$. A gauge change transfers the flux to a <twisted boundary condition> rather than removing it from the physical problem. Half-integer $f$ gives a pair of degenerate nearest-momentum levels.
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