If a charged particle travels in a field free region that surrounds another region, in which there is trapped magnetic flux F, then upon completing a closed loop the particles wave function will acquire an additional phase factor But the wave function must be single valued at any point in space. This can be accomplished if the magnetic flux F is quantized. We need
This quantization of the magnetic flux is observed in superconductors. Superconductivity is theorized to be due to a special correlation between pairs of electrons that extends over the whole body of the superconductor. When a Type I superconductor is placed in a magnetic field and cooled below its critical temperature, it excludes all magnetic flux from its interior. This is called the Meissner effect. If there is a "hole" in the superconductor, then flux can be trapped in this hole. The flux trapped in the hole must be quantized. It has been experimentally verified that the trapped flux is quantized in units of thus verifying that the charge carriers in superconductors are indeed correlated electron pairs of charge 2e.
In Maxwells equations magnetic charges do not appear. We have . No magnetic charges have been confirmed to exist. Quantum mechanics does not require that magnetic charges exist, but it unambiguously requires the quantization of magnetic monopoles and predicts the unit of magnetic charge if they should ever be found.
Assume that magnetic monopoles exist and that a magnetic monopole is located at the origin. Then and
A possible solution is A=eM(1-cosq)/(rsinq) in the f-direction. But A is singular on the negative z-axis at q=p. If we consider A just a device for obtaining B, then we can construct a pair of vector potentials
which together yield the correct B everywhere. A1 can be used everywhere except inside a cone defined by q=p-e around the negative z-axis, and A2 can be used everywhere except inside a cone defined by q=e around the positive z-axis. In the overlap region either A1 or A2 can be used. The two potentials lead to the same magnetic field and are therefore related to each other by a gauge transformation.
The wave function of a charged particle depends on the particular gauge used. In the overlap region we have Here e is the particle's electric charge. The wave function must be single valued. As we increase the azimuthal angle f from 0 to 2p, the wave function must return to its original value. This is only possible if We therefore find that eM must be quantized in units of The smallest magnetic charge possible is
change the Schroedinger equation and therefore change the wave function. But they change both in such a way that the results of a measurement do not change.
However, knowing that the phase of the wave function depends on the gauge and the physical predictions do not depend on the gauge, we can reach some interesting conclusions.