  It is shown that the magnetic energy of a quantum current, contrary to the
classical case, is essentially negative. Since this result allows to escape a
famous theorem by Bloch, it can be expected that, under appropriate conditions,
the ground state of a quantum conductor may be characterized by a spontaneous
current. A similar idea was suggested, many years ago, by Heisenberg and by
Born and Cheng as a possible mechanism for superconductivity.
