Superconducting qubits realized in macroscopic-scale electric circuits can be used as artificial atoms with two or more quantized levels. Being man-made quantum systems, they offer us large flexibility in designing their properties, such as transition energies, transition matrices, selection rules, spatial positions, and coupling strengths to other parts of the circuits, etc. We have investigated on-chip microwave-regime quantum optical properties of an artificial atom coupled to a continuum, i.e., a flux qubit coupled to a 1D microwave transmission line. The strong coupling, now characterized by the large spontaneous emission rate, as well as the spatial mode matching between the incident and emitted modes allow us to observe various quantum optical phenomena.

[1] O. Astafiev et al., Science 327, 840 (2010).
[2] O. Astafiev et al., PRL 104, 183603 (2010).
[3] A. A. Abdumalikov, Jr. et al., PRL 104, 193601 (2010).