From cavity to circuit quantum electrodynamics

This article puts in perspective the relationship between cavity and circuit quantum electrodynamics, two related approaches for studying the fundamental quantum interaction between light and matter. Circuit quantum electrodynamics focuses on the interaction of small superconducting circuits, tailored to behave as two-level quantum systems, with a single mode of the electromagnetic field sustained by a superconducting resonator. It is thus concerned with the investigation of phenomena that arise from the coupling between the simplest non-trivial quantum system - a spin-1/2 or qubit - and a harmonic oscillator. As such, circuit quantum electrodynamics belongs to the more general field of cavity quantum electrodynamics, which deals with natural or artificial spins in the optical, microwave or radio-frequency domains interacting with all kind of resonators. Here we survey the lineage of the concepts and experiments that led first to the development of cavity and then circuit quantum electrodynamics. We discuss similarities and differences between these two fields and compare their present achievements.