Physical Society Colloquium

Circuit QED: Quantum Optics and Quantum Computing on a Superconducting Chip

Robert Schoelkopf

Yale University

When is an electrical signal a photon? I will describe a system we call circuit quantum electrodynamics, which is a way to do new experiments in quantum optics with a superconducting integrated circuit. In circuit QED, microwave photons are guided and confined by superconducting transmission lines and cavities, and can then be coherently coupled to a Cooper-pair box, which is a kind of two-level &ldquoartificial atom&rdquo or qubit made with Josephson junctions. This system leads to much stronger coupling of the &ldquolight&rdquo and &ldquomatter&rdquo than are possible with traditional atomic systems. We can access the usual strong coupling limit of cavity QED, as well as a new &ldquostrong dispersive&rdquo regime where the qubit and photon interact without exchanging energy. Here it is possible to perform quantum non-demolition measurements of both the qubit and the photon, and we see dramatic evidence for the energy quantization of single microwave (5 GHz) photons. The prospects for true single photon generation and measurement, and the advantages of this system for quantum information processing, will also be discussed.