  We consider a quantum mechanical model for the high-order harmonic generation
in bulk solids. The bandgap is assumed to be considerably larger than the
exciting photon energy. Using dipole approximation, the dynamical equations for
different initial Bloch states are decoupled in the velocity gauge. Although
there is no quantum mechanical interference between the time evolution of
different initial states, the complete harmonic radiation results from the
interference of fields emitted by all the initial (valence band) states. In
particular, this interference is shown to be responsible for the suppression of
the even order harmonics. The number of the observable harmonics (essentially
the cutoff) is also determined by electromagnetic field interference.
