In this talk, I will address advances on quantum optics with superconducting artificial atom.
In the first sets of experiments, we embed a transmon artificial atom in an open transmission line. When a weak coherent state is on resonance with the atom, we observe extinction of up to 99% in the forward propagating field. By applying a second control tone, we observe the Autler-Townes splitting, we demonstrate operation of a single-photon router  using the Autler-Townes splitting. In the second sets of experiment, we embed a transmon at a distance from the end (mirror) of a transmission line . By tuning the wavelength of the atom, we effectively change the normalized distance between atom and mirror, allowing us to effectively move the atom from a node to an antinode of the vacuum fluctuations. We probe the strength of vacuum fluctuations by measuring spontaneous emission rate of the atom. In the third sets of experiments, we strongly couple a transmon to the field in a semi-infinite waveguide. When driving the qubit strongly on resonance such that a Mollow triplet appears, we observe a 7% amplitude gain for a weak probe at frequencies in between the triplet . This amplification is not due to population inversion, but instead results from a four-photon process that converts energy from the strong drive to the weak probe.
 I.-C. Hoi et al. Physical Review Letters 107, 073601 (2011)
 I.-C. Hoi et al. Nature Physics 11, 1045 (2015)
 P. Y. Wen et al. Physical Review Letters 120, 063603 (2018)