Abstract
It is found by computer simulation that in the atomic beam deflection experiment proposed by Holland et al. to perform a quantum non-demolition measurement of the photon number, the rate of entropy decrease owing to the transition to a Fock state depends in a quasi-periodic way on the interaction parameter. Moreover, a simple analytic expression is derived for the decay of the off-diagonal field density matrix elements. With the help of this expression the phase destruction of the field due to the measurement process is investigated.