  In this paper, we propose a resources-optimal linear-optical scheme for
quantum nondemolition detection of single-photon presence. By measuring the
state of ancillary photons, the presence of a photon in signal mode is revealed
with a success probability of 1/2 without any disturbance to its state. We also
show how to tune the setup to perform quantum nondemolition measurement of the
signal photon state, and we provide tradeoff between the extracted information
and the signal state disturbance. Moreover, the optimality of resources and
methods by which to increase the success probability are discussed.
