  The electromagnetic field inside a spherical cavity of large radius R is
considered in the presence of stationary charge and current densities. R
provides infra-red regularization while maintaining gauge invariance. The
quantum ground state of physical photons forming the magnetic field is found to
be a coherent state with a definite mean occupation number. The electric field,
which is determined by the Gauss law constraint, is maintained by a minimum
uncertainty coherent state, according to the projection operator approach to
the quantization of constrained systems. The mean occupation number of this
state is proportional to the square of the total charge. The results confirm
formulae obtained previously from a calculation with a finite photon mass for
infra-red regularization.
