  The paper discusses the single-mode Jaynes-Cummings model with time dependent
parameters. Solvable models for two-level systems are utilized to consider the
changes in the photon distribution affected by the passage of atoms through the
cavity. It is suggested that such systems may be used as filters to modify the
photon distribution. The effect can be enhanced by repeatedly sending new atoms
through the cavity. We show that such filters can cut out either small or large
photon numbers. It is also shown that the method can be used to narrow down
photon distributions and in this way achieve highly non-classical
sub-Poissonian states. Some limitations and applications of the method are
presented.
