Circuit QED: Recent Results in Quantum Optics with Superconducting Circuits

Circuit QED1 is an approach for studying quantum optics in a superconducting integrated circuit. By combining a one-dimensional transmission-line cavity that stores microwave photons and a superconducting qubit that plays the role of an artificial atom, one can easily enter the strong coupling limit of cavity QED. In recent experiments, we attain couplings that are several percent of the qubit or cavity frequency, and in fact approach the maximal fine-structure limit for a electric-dipole interaction of light and matter, giving rise to a remarkable vacuum Rabi splitting of several hundred linewidths. We will present studies of the nonlinear response of this system, which shows two novel effects: 1) each vacuum Rabi peak develops a supersplitting, which can be understood in a simple picture as the saturation of a new two-level system consisting of photon-qubit superpositions, and 2) the emergence of additional peaks, corresponding to multi-photon transitions up the JaynesCummings ladder, and constituting a simple demonstration of the √