This thesis discusses the realization of a solid-state, electrical circuit analog of atomic cavity quantum electrodynamics in which a superconducting charge qubit is strongly coupled to a coherent microwave radiation field contained in a high quality factor transmission line resonator, an architecture that has become to be known as circuit quantum electrodynamics (QED). This system allows studying the interaction of individual photons with an artificial atom and to perform fundamental quantum optics experiments on an electrical chip.
Moreover, it has a strong, demonstrated potential for the implementation of a solid-state quantum information processor.
In the framework of this thesis, two major experiments have been carried out in the circuit QED architecture. First, on-chip quantum optics experiments are presented in which we were able to investigate the interaction of matter and light down to the level of single quanta, ultimately reaching the limit where a superconducting qubit dispersively interacts with the pure vacuum fluctuations of the electromagnetic cavity field. Here, the presence of virtual photons manifests itself as a small renormalization of the energy of the qubit in the form of the Lamb shift, which is observed in our experiments for the first time in a solid-state system. The transition from the strong dispersive to the resonant strong interaction is shown as a smooth, continuous overlap of cavity transmission and qubit spectroscopy data, in a region where the Lamb shift turns into the vacuum Rabi splitting of the cavity-qubit superposition states. The enhancement by the cavity leads to maximum observable Lamb shifts of up to 1.4% relative to the qubit transition frequency. Following the vacuum field measurements, the photon number is subsequently increased. In this regime, some interesting results will be presented that allow us to put the Lamb shift in a consistent picture and to resolve individual photon number states of the cavity.
The second part of this thesis focuses on the development of a novel type of transmission line resonator. As opposed to conventional circuit QED cavities, the resonance frequency of these devices can be dynamically tuned with an external magnetic field. Flux-tunable transmission line resonators are systematically investigated in this thesis, ranging from a thorough theoretical analysis to detailed experimental studies in which the tuning behavior of both the resonance frequency as well as the quality factor is determined. Large tuning ranges of up to 2.5 GHz are observed and it is shown that the behavior of the quality factor can be controlled by choosing adequate design parameters. Finally, first experiments are presented in which we demonstrate the successful strong coupling of a superconducting transmon qubit to a flux-tunable cavity.