  Using time correlated single photon counting combined with temperature
dependent diffraction limited confocal photoluminescence spectroscopy we
accurately determine, for the first time, the intrinsic radiative lifetime of
single excitons confined within semiconductor quantum dots. Their lifetime is
one (two) orders of magnitude longer than the intrinsic radiative lifetime of
single excitons confined in semiconductor quantum wires (wells) of comparable
confining dimensions. We quantitatively explain this long radiative time in
terms of the reduced spatial coherence between the confined exciton dipole
moment and the radiation electromagnetic field.
