A Brief History of Circuit QED and Superconducting Quantum Information Processing

Circuit QED is the quantum electrodynamics of superconducting microwave circuits and is today the leading architecture for development of quantum information processors based on superconducting qubits. The study of superconducting electrical circuits began in the 1960’s with the observation of the Josephson effect. Ideas by Anthony Leggett about macroscopic quantum tunneling led to remarkable experiments in the Clarke group at Berkeley in the 1980’s demonstrating that the superconducting order parameter was a quantum mechanical degree of freedom exhibiting energy level quantization, and quantum tunneling. This in turn led to the invention of numerous species of superconducting qubits which have advanced in the last 25 years by a factor of nearly 10⁶ in coherence time. These advances have opened up a completely new regime of strong-coupling non-linear optics in the microwave domain whose parameters and capabilities are dramatically different from traditional optical cavity QED. They have also led to enormous progress in industrial scaling up of the technology and have brought us into the beginnings of the era of quantum error correction. We may now be standing close to the boundary of the NISQ era and the coming era of fault-tolerant quantum machines.