Photon blockade effect in the ultrastrong coupling regime

One of the most prominent effects that produces emission of nonclassical light is a phenomenon known as photon blockade, where a classical coherent excitation of a cavity coupled to a highly nonlinear quantum system, such as a single atom, quantum dot or superconducting qubit, generates an output train of single photons. Here we present calculations for photon coincidence counting statistics in the ultrastrong coupling regime, where the strength of the interaction between an emitter and the cavity photons becomes comparable to the transition frequency of the emitter or the resonance frequency of the cavity mode. In such a regime of interaction, the usual normal order correlation functions [1] fail to describe the output photon statistics. Exploiting the correct input-output relations within a suitable Master Equation approach [2,3], we are able to propose correlation functions that are valid for arbitrary degrees of light-matter interaction. © 2013 IEEE.