  For spin-based quantum computation in semiconductors, dephasing of electron
spins by a fluctuating background of nuclear spins is a main obstacle. Here we
show that this nuclear background can be precisely controlled in generic
quantum dots by periodically exciting electron spins. We demonstrate this
universal phenomenon in many-electron GaAs/AlGaAs quantum dot ensembles using
optical pump-probe spectroscopy. A feedback mechanism between the saturable
electron spin polarization and the nuclear system focuses the electron spin
precession frequency into discrete spin modes. Employing such control of
nuclear spin polarization, the electron spin lifetime within individual dots
can surpass the limit of nuclear background fluctuations, thus substantially
enhancing the spin coherence time. This opens the door to achieve long electron
spin coherence times also in lithographically-defined many-electron systems
that can be controlled in shape, size and position.
