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arXiv:1807.11636 · Nature Reviews Physics

Ultrastrong coupling between light and matter

Anton Frisk Kockum, Adam Miranowicz, Simone De Liberato, Salvatore Savasta, Franco Nori

Published Jan 3, 2019• 1,470 citations
arXivHTMLPDF
DOI
10.1038/s42254-018-0006-2

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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.