  We consider realistic photodetection in a generalization of the
Hong-Ou-Mandel experiment to the multimode case. The basic layout of this
experiment underlies boson sampling -- a promising model of nonuniversal
quantum computations. Peculiarities of photocounting probabilities in such an
experiment witness important nonclassical properties of electromagnetic field
related to indistinguishability of boson particles. In practice, these
probabilities are changed from their theoretical values due to the imperfect
ability of realistic detectors to distinguish numbers of bunched photons. We
derive analytical expressions for photocounting distributions in the
generalized Hong-Ou-Mandel experiment for the case of realistic photon-number
resolving (PNR) detectors. It is shown that probabilities of properly
postselected events are proportional to probabilities obtained for perfect PNR
detectors. Our results are illustrated with examples of arrays of on/off
detectors and detectors affected by a finite dead time.
