Fig. 11. Diagram representing spontaneous production of correlated exciton–photon pairs from the bare system ground-state |0Diagram representing spontaneous production of correlated exciton–photon pairs from the bare system ground-state |0〉. This process is significant in the ultrastrong coupling regime where light-matter couplings of the form . This process is significant in the ultrastrong coupling regime where light-matter couplings of the form 
become relevant (see eqn (15)).
