  We use temporally resolved intensity cross-correlation measurements to
identify the biexciton-exciton radiative cascades in a negatively charged QD.
The polarization sensitive correlation measurements show unambiguously that the
excited two electron triplet states relax non-radiatively to their singlet
ground state via a spin non conserving flip-flop with the ground state heavy
hole. We explain this mechanism in terms of resonant coupling between the
confined electron states and an LO phonon. This resonant interaction together
with the electron-hole exchange interaction provides an efficient mechanism for
this, otherwise spin-blockaded, electronic relaxation.
