  Here one can find a derivation of the commutator of the charge and magnetic
flux in a superconducting circuit containing a Josephson junction from the
commutation relation of quantum-field operators of electric field and vector
potential. A commutator of the fluxes of electric and magnetic fields passing
through two interlinked loops in vacuum is also evaluated and physical
interpretation of the commutator is discussed.

Quantum Physics;High Energy Physics - Theory