  Single-photons play an important role in emerging quantum technologies and
information processing. An efficient generation technique consists in preparing
such states via a conditional measurement on photon-number correlated beams:
the detection of a single-photon on one of the beam can herald the generation
of a single-photon state on the other one. Such scheme strongly depends on the
heralding detector properties, such as its quantum efficiency, noise or
photon-number resolution ability. These parameters affect the preparation rate
and the fidelity of the generated state. After reviewing the theoretical
description of optical detectors and conditional measurements, and how both are
here connected, we evaluate the effects of these properties and compare two
kind of devices, a conventional on/off detector and a two-channel detector with
photon-number resolution ability.
