Photon Blockade in the Ultrastrong Coupling Regime

TL;DR

This paper investigates photon statistics in the ultrastrong light-matter coupling regime, revealing significant modifications to photon blockade and new phenomena like parametric processes and oscillations due to counter-rotating interactions.

Contribution

It introduces a new approach to define correlation functions valid in ultrastrong coupling, addressing limitations of traditional methods.

Findings

Photon blockade is altered in the ultrastrong regime.

Parametric processes occur even with two-level emitters.

Temporal oscillations of correlation functions are observed.

Abstract

We explore photon coincidence counting statistics in the ultrastrong-coupling regime where the atom-cavity coupling rate becomes comparable to the cavity resonance frequency. In this regime usual normal order correlation functions fail to describe the output photon statistics. By expressing the electric-field operator in the cavity-emitter dressed basis we are able to propose correlation functions that are valid for arbitrary degrees of light-matter interaction. Our results show that the standard photon blockade scenario is significantly modified for ultrastrong coupling. We observe parametric processes even for two-level emitters and temporal oscillations of intensity correlation functions at a frequency given by the ultrastrong photon emitter coupling. These effects can be traced back to the presence of two-photon cascade decays induced by counter-rotating interaction terms.