  We report optical pumping of neutral quantum dots leading to nuclear spin
alignment with direction insensitive to polarization and wavelength of light.
Measurements of photoluminescence of both "dark" and "bright" excitons in
single dots reveal that nuclear spin pumping occurs via a virtual spin-flip
transition between these states accompanied by photon emission. The sign of the
nuclear spin polarization is determined by asymmetry in the exciton energy
spectrum, rather than by the sign of the exciton spin polarization.
