Magnetic energy of a quantum current

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.