I propose that the phase of an electron's wave function changes by π when
the electron goes around a loop maintaining phase coherence. Equivalently, that
the minimum orbital angular momentum of an electron in a ring is ℏ/2
rather than zero as generally assumed, hence that the electron in a ring has
azimuthal zero point motion. This proposal provides a physical explanation for
the origin of electronic `quantum pressure', it implies that a spin current
exists in the ground state of aromatic ring molecules, and it suggests an
explanation for the ubiquitousness of persistent currents observed in
mesoscopic rings