We also recommend to readers the following resources for further reading. Ref. 42 provides an excellent review on different coupling regimes of light-matter interactions, including the ultra-strong and deep-strong couplings. Ref. 43 provides an extensive discussion on gauge ambiguities in a broader perspective. Ref. 44 provides a thorough review on recent progress in molecular cavity QED. Refs. 45-49 provide fundamental discussions on QED and cavity QED. Lastly, Refs. 48,50 provide an excellent introduction to quantum optics.