Announcing single photons from imperfect sources by means of “interaction-free" measurements

. Interaction-free measurements can indicate the presence of photo-sensitive objects and even image the objects without damaging them. Can they also be used to indicate the presence of a single photon without absorbing it? We address this question in the context of improving the efficiency of single-photon sources. In particular, we interpret a recently proposed method to announce single photons based on two-photon absorption [Konrad et al., Phys. Rev. A 73 , 032327 (2006)] in terms of "interaction-free" measurements.