  We present an analytic, and fully quantum theory of high harmonic generation
by low frequency laser fields for a two-state hydrogenlike atom model. The
model is valid for an arbitrary atomic potential role of dipole matrix elements
and can be generalized to describe laser fields of arbitrary polarization and
includes the depletion of states. In the case of an initial coherent
superposition of two states, the theory clearly explains why high conversion
efficiency can be obtained. Our results are based on quantum interference
effects in recombination via different states. We demonstrate that four quantum
paths of electron contribute in the harmonic spectrum. The harmonic conversion
efficiency is directly proportional to the initial-state amplitudes and
depletion of the states (especially excited state), the remained population of
the bound states at the time of recombination and the distinct dipole
strengths. The main contribution to high conversion efficiency is the
transition from the excited to the ground state.
