  The manipulation of individual, mobile quanta is a key goal of quantum
communication; to achieve this, nonlinear phenomena in open systems can play a
critical role. We show theoretically that a variety of strong quantum nonlinear
phenomena occur in a completely open one-dimensional waveguide coupled to an
N-type four-level system (4LS). We focus on photon blockade and the creation of
single photon states in the absence of a cavity. Many-body bound states appear
due to the strong photon-photon correlation mediated by the 4LS. These bound
states cause photon blockade which can generate a sub-Poissonian single photon
source.
