  New processes associated with the nonlinear optical properties of the
electromagnetic vacuum, as predicted by quantum electrodynamics are described.
We consider the presence of a static and a rotating magnetic field. The cases
of harmonic generation and two different types of sideband cascades are
considered. First order and second order effects with respect to the external
magnetic field are discussed in detail. Possible experimental configurations
using ultra-intense lasers are compared with the alternative use of cw lasers
in stable optical cavities.
