cQED

The combination of microwave photons with superconducting quantum circuits offers promise for quantum technologies and the fundamental study of quantum light–matter interactions. This month, a Focus issue explores this field of research.

In 1985, experiments revealed the quantum behaviour of a macroscopic degree of freedom: the phase difference across a Josephson junction. The authors recount the history of this milestone for the development of superconducting quantum circuits.

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.

The introduction of concepts from cavity quantum electrodynamics to superconducting circuits yielded circuit quantum electrodynamics, a platform eminently suitable to quantum information processing and for the exploration of novel regimes in quantum optics.

Hybrid quantum systems combine heterogeneous physical systems for the implementation of new functionalities at the quantum level. This article reviews recent research on the creation of hybrid quantum systems within the circuit quantum electrodynamics framework.

This Review Article surveys the physics of many-body quantum states formed by microwave photons in circuit quantum electrodynamics environments.