Researchers in many fields of physics have envisaged to couple a single quantum two-level system coherently to a quantum harmonic oscillator, see A. Blais et al. and references therein. Until recently, this feat had only been achieved in the realm of atomic physics and quantum optics, where a single atom in a ultra high vacuum is observed to coherently exchange a single photon with a single mode of a radiation field in atomic cavity quantum electrodynamics (QED) experiments. Such experiments have been at the focus of research in atomic physics and quantum optics in the past decade and have contributed greatly to our fundamental understanding of the interaction of matter with quantized electromagnetic fields, of the physics of open quantum systems and of coherence and decoherence. They are also a major test ground for developing and demonstrating the general concepts of quantum information processing.

In a recent experimental breakthrough, covered in the September 9, 2004 issue of Nature (A. Wallraff et al.) and in the November 2004 issue of Physics Today (M. Wilson), our group at Yale has demonstrated for the first time that a solid-state nano-electronic superconducting circuit, acting as an artificial atom, can be coupled coherently to a single microwave photon stored in an on-chip cavity. The success of this approach is based on our ability to design and fabricate mesoscopic solid-state two-level systems with controllable properties, such as large effective electric dipole moments (d ~ 10⁴ e a₀), and to develop 1D on-chip cavities with small mode volumes and very large vacuum electric fields (E₀ ~ 0.1 V/m) leading to large controllable electric dipole interactions. Key is also the development of microwave techniques for experiments with mesoscopic devices at low temperatures. Mastering these techniques, we have realized this new paradigm of circuit quantum electrodynamics. In circuit QED coherent quantum optical effects can be observed even in a solid state environment where coupling to environmental degrees of freedom leading to decoherence is typically much larger than in atomic systems.

The circuit QED system developed during my time at Yale is extremely attractive for fundamental cavity QED experiments. Its properties can be designed at will in integrated circuit fabrication and tuned in situ in experiment while allowing for extremely large coupling strengths. At the same time our artificial atom is deterministically at a fixed position within the cavity, avoiding fluctuations in the coupling strength. These are major advantages in comparison to atomic cavity QED. Thus this circuit QED system has great prospects to enable experiments that complement existing work and go beyond the current state of the art of atomic cavity QED. We plan to address fundamental quantum optics questions, applications in quantum computation and the use of this architecture as a sensitive measurement device or detector in our work.