Announcing single photons from imperfect sources by means of “interaction-free" measurements T. Konrad, A. Scherer, M. Nock and J. Audretsch Eur. Phys. J. Special Topics, 159 (2008) 93-99 Published online: 21 May 2008 DOI: 10.1140/epjst/e2008-00697-7