  We consider the time-delayed coincidence counting of two photons emitted in a
cascade by a single particle (atom, molecule, nucleus, etc). The
time-dependence of the probability amplitude of the second photon in the
cascade has a sharply rising leading edge due to the detection of the first
photon, as results from causality. If a macroscopic ensemble of resonant
two-level absorbers is placed in the path of the second photon between the
radiation source and the detector, the photon absorption does not follow Beer's
law due to the time-asymmetric shape of the photon. For very short delay times
almost no absorption takes place, even in an optically dense medium. We analyze
the propagation of such a second photon in a thick resonant three-level
absorber if a narrow electromagnetically induced transparency (EIT) window is
present at the center of the absorption line. It is shown that the EIT medium
can change the asymmetric time dependence of the photon probability amplitude
to a bell shape (EIT filtering). This bell-shaped photon interacts much more
efficiently with an other ensemble of two-level absorbers chosen, for example,
to store this photon and the information it carries.
