This lecture covers the operation of circuits quantum mechanically at ultra-low temperatures, the detection of microwave fields at the single photon level, and the interaction of atoms and photons in a cavity. It explains cryogenic and room temperature measurement setups, cryogenic amplification, and filtering, as well as controlling light-matter interactions challenging on the level of single atoms and photons. The lecture also delves into cavity quantum electrodynamics, the Jaynes-Cummings Hamiltonian, and the observation of vacuum Rabi splitting in electrical circuits.