  The relation between single particle and ensemble measurements is adressed
for semiconductor CdSe nanocrystals. We record their fluorescence at the single
molecule level and analyse their emission intermittency, which is governed by
unusual random processes known as Levy statistics. We report the observation of
statistical aging and ergodicity breaking, both related to the occurrence of
Levy statistics. Our results show that the behaviour of ensemble quantities,
such as the total fluorescence of an ensemble of nanocrystals, can differ from
the time averaged individual quantities, and must be interpreted with care.
