  Propagation of a two-photon pulse in a waveguide coupled to a two-level
system (TLS) is studied. The pulse is formed by two spatially separated
identical wavepackets. A set of equations governing the dynamics of the photon
distribution in the configuration-momentum space is derived and solved. It is
shown that the distribution function can be negative that manifests its
quasiprobability nature. A spectrum of the reflected light is found to be
narrower than that of the transmitted light that features a pronounced
filtering effect. Average numbers of the transmitted and reflected photons and
their variances are shown to be dependent not only on the pulse widths and the
light-TLS interaction but also on the pulse separation that can serve as an
effective controlling parameter. Our approach is generalized for the case of an
n-photon Fock state.
