Atomic-beam and cavity-field correlation functions in the micromaser

It is shown that the autocorrelation function of the current produced through the ionization of ground-state atoms emerging from a micromaser cavity can be expressed, apart from a shot-noise contribution, in terms of a two-time correlation function of cavity-field variables. Numerical comparisons between this latter correlation function and the intensity correlation function of the cavity field show that the two are closely related and, in fact, in the limit of a zero-temperature cavity-field reservoir, become identical. A proof of this last result is presented.