Correlating Photons Using the Collective Nonlinear Response of Atoms Weakly Coupled to an Optical Mode

The generation of correlated photons typically requires a highly nonlinear medium that is strongly coupled to an optical mode. Recently, significant advances have been made in this direction by strongly coupling photons to quantum emitters and exploiting the inherently nonlinear response of the latter. A number of methods have been pursued for this purpose, employing, e.g., high finesse optical cavities or strongly interacting Rydberg atoms. However, the implementation of strong and low-loss photon-photon interactions remains a challenging goal.