  By discussing field-induced quantum interference effects due to monopole
moments and those due to dipole moments on equal footing, their similarities
and differences are clarified. First, we demonstrate the general principle for
flux quantization. For particles carrying a monopole moment, the interference
causes monopole current to oscillate periodically with flux defined as inner
product of field and area, whereas for particles carrying a fixed dipole
moment, the dipole current oscillates periodically with flux vector defined as
cross product of field and trajectory. Our analysis unifies the oscillation of
monopole or dipole currents in various devices, such as SQUID and spin-FET,
into the same physical picture. Second, we show that interference effects can
also happen in open trajectory devices that transport dipole currents, such as
spin Josephson effect, based on the non-gauge field nature of the interference
effects of dipole moments. In addition, we propose that the interference effect
of electric dipoles, known as He-McKellar-Wilkens effect, can be realized by
the bilayer exciton condensates observed in semiconductor heterostructure and
bilayer graphene.
