  The periodical modulation of circularly polarized light with a frequency
close to the electron spin resonance frequency induces a sharp change of the
single electron spin orientation. Hyperfine interaction provides a feedback,
thus fixing the precession frequency of the electron spin in the external and
the Overhauser field near the modulation frequency. The nuclear polarization is
bidirectional and the electron-nuclear spin system (ENSS) possesses a few
stable states. A similar frequency-locking effect exists for two-color and
mode-locked excitations, too. However, the pulsed excitation with mode locked
laser brings about the multitudes of stable states in ENSS in a quantum dot.
The resulting precession frequencies of the electron spin differ in these
states by the multiple of the modulation frequency. Under such conditions ENSS
represents a digital frequency converter with more than a hundred stable
channels.
