  We report measurements of the time for spin flip from dark (non-light
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 approximately 200 ps, independent of density and temperature below
70 K. This is much shorter than observed in other quantum dot systems. The rate
of decay of the luminescence intensity, approximately 300 ps, is not simply
equal to the radiative decay rate from the bright states, because the rate of
decay is limited by the rate of conversion from dark excitons into bright
excitons. The dependence of the luminescence decay time on the spin flip time
is a general effect that applies to many experiments.
