Ultrastrong Coupling Signatures in Photon Statistics from Terahertz Higgs-Polaritons

TL;DR

This paper demonstrates that photon statistics can reveal signatures of ultrastrong light-matter coupling in cavity-embedded superconductors, providing a new diagnostic tool for this regime.

Contribution

It introduces a method to detect ultrastrong coupling via two-photon photon statistics, highlighting the formation of a hybrid photon-matter dark-cavity state.

Findings

Photon statistics change significantly with increased light-matter coupling.

Formation of a hybrid photon-matter dark-cavity state with finite photon occupancy.

A diagnostic for ultrastrong coupling based on two-photon coincidence statistics.

Abstract

The ultrastrong coupling regime of cavity photons and quantum materials has emerged as a pathway to modify materials properties, however definitive signatures of ultrastrong coupling remain elusive. Focusing on the quantum photon statistics of light transmitted through a cavity-embedded superconductor, we show that a two-photon Higgs polariton at strong coupling realizes a photonic nonlinearity at the single terahertz photon level. We find that as light-matter coupling increases, the photon statistics show pronounced changes due to the formation of a hybrid photon-matter dark-cavity state with finite photon occupancy, producing testable signatures of ultrastrong coupling. We derive a non-Markovian input output relation and study the cavity-embedded superconductor 2H-NbSe2 as it approaches ultrastrong light-matter coupling. Our results reveal a diagnostic for ultrastrong coupling in the two-photon coincidence statistics that is absent in total counts.