Spin flip from dark to bright states in InP quantum dots

We report measurements of the time for spin flip from dark (nonlight emitting) exciton states in quantum dots to bright (light emitting) exciton states in InP quantum dots. Dark excitons are created by two-photon excitation by an ultrafast laser. The time for spin flip between dark and bright states is found to be at most $200\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, independent of density and temperature, below $70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This is much shorter than observed in other quantum dot systems.