Abstract: The non-linear interaction of light with matter, long studied in the context of atomic systems, has recently been extended to condensed matter systems through the advent of quantum optical experiments based on superconducting circuits. Application of quantum optical techniques in this context has led to new regimes of ultrastrong coupling between light and matter, manipulation and readout of qubits, generation of quantum states of light and development of ultra-low-noise quantum amplifiers. A particularly familiar application of quantum optics is the laser, which in the single emitter regime has in atomic systems led to the production of pure photon number states and sub-Poissonian photon statistics. We have recently produced a device consisting of a Cooper pair transistor embedded in a high-Q superconducting microwave cavity (cCPT) that acts as a single emitter laser and may offer a path toward simple, continual production of non-classical photons. Similar devices may also allow for ultrastrong coupling of microwave photons to other quantum systems such as spin qubits and nanomechanical resonators.