  The statistics of photons emitted by single multilevel systems is
investigated with emphasis on the nonrenewal characteristics of the
photon-arrival times. We consider the correlation between consecutive
interphoton times and present closed form expressions for the corresponding
multiple moment analysis. Based on the moments a memory measure is proposed
which provides an easy way of gaging the non-renewal statistics. Monte-Carlo
simulations demonstrate that the experimental verification of non-renewal
statistics is feasible.
