  We study the generation of strongly correlated photons by coupling an atom to
photonic quantum fields in a one-dimensional waveguide. Specifically, we
consider a three-level or four-level system for the atom. Photon-photon bound
states emerge as a manifestation of the strong photon-photon correlation
mediated by the atom. Effective repulsive or attractive interaction between
photons can be produced, causing either suppressed multiphoton transmission
(photon blockade) or enhanced multiphoton transmission (photon-induced
tunneling). As a result, nonclassical light sources can be generated on demand
by sending coherent states into the proposed system. We calculate the
second-order correlation function of the transmitted field and observe bunching
and antibunching caused by the bound states. Furthermore, we demonstrate that
the proposed system can produce photon pairs with a high degree of spectral
entanglement, which have a large capacity for carrying information and are
important for large-alphabet quantum communication.
