  We find that detuning an optical pulse train from electronic transitions in
quantum dots controls the direction of nuclear spin flips. The optical pulse
train generates electron spins that precess about an applied magnetic field,
with a spin component parallel to the field only for detuned pulses. This
component leads to asymmetry in the nuclear spin flips, providing a way to
produce a stable and precise value of the nuclear spin polarization. This
effect is observed using two-color, time-resolved Faraday rotation and
ellipticity.
