Quantum rotor
= Quantum rotor
{title2=$H=\ell^2/(2I),\quad E_n=\hbar^2n^2/(2I)$}
A quantum coordinate on a circle with kinetic Hamiltonian $\ell^2/(2I)$. Periodic wavefunctions have angular momentum $\hbar n$ and energy $\hbar^2n^2/(2I)$ for integer $n$. Its thermal trace has both a momentum-eigenstate sum and a topological winding-path representation. Twisted boundary conditions or flux would modify this spectrum and are separate models.