Probing ultrastrong light–matter coupling in open quantum systems A. Ridolfo, J. Rajendran, L. Giannelli, E. Paladino and G. Falci Eur. Phys. J. Spec. Top., 230 4 (2021) 941-945 Published online: 06 May 2021 DOI: 10.1140/epjs/s11734-021-00070-8