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 $2{\mathrm{H}\text{−}\mathrm{NbSe}}_2$ 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.

Close

PrevNext

View figure in article

• Cavity quantum electrodynamics
• Quantum optics
• Quantum cavities
• Superconductors
• Non-Markovian processes