  Mean values of some observables describing quantum interaction between the
Bose field in a cavity and a movable mirror can be represented as expectations
of rapidly oscillating functions w.r.t. the Poisson measure with a large mean
value ($N\approx 10^{23}$) corresponding to the average number of photons in
laser beam. Straightforward summation of the series is impossible because over
$2\sqrt N$ summands make a significant contribution. We derive an analytical
expression approximating this sum with the error $O(N^{-1})$.
