The Mandel Q parameter measures the departure of the occupation number distribution from Poissonian statistics.
[1] It is a convenient way to characterize non-classical states with negative values indicating a sub-Poissonian statistics, which have no classical analog.
is the normalized second-order correlation function as defined by Glauber.
[2] Negative values of Q corresponds to state which variance of photon number is less than the mean (equivalent to sub-Poissonian statistics).
The resulting occupation distribution of the number state is characterized by a Bose–Einstein statistics for which