  We show that two-photon transport is strongly correlated in one-dimensional
waveguide coupled to a two-level system. The exact S-matrix is constructed
using a generalized Bethe-Ansatz technique. We show that the scattering
eigenstates of this system include a two-photon bound state that passes through
the two-level system as a composite single particle. Also, the two-level system
can induce effective attractive or repulsive interactions in space for photons.
This general procedure can be applied to the Anderson model as well.
