Cavity-free Photon Blockade Induced by Many-body Bound States
  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. 
Huaixiu Zheng, Daniel J. Gauthier, Harold U. Baranger
cond-mat.mes-hall physics.atom-ph quant-ph