The simplest quantum model of an atom interacting with light is that of a two-level system (TLS) interacting with a single or multi-mode electromagnetic environment. The intrinsic nonlinearity of a TLS allows this model to capture a wide breadth of quantum optics phenomena, from the emission of anti-bunched light under strong driving, to ultrastrong coupling with artificial atoms. Nowadays the quantum properties of single TLSs can be investigated on many different physical platforms beyond atoms, including trapped ions and NV centers, quantum dots and superconducting circuits [1, 2].