Quantum optics in circuit quantum electrodynamics with a tunable four-level system in the nesting regime

Circuit quantum electrodynamics (QED) is a paradise for studying quantum optics in the microwave domain because it offers highly scalable architecture with long-lived artificial atoms, high quality resonators, large nonlinearity, and tunability. Although some remarkable quantum optics experiments have been implemented in circuit QED system, more exciting phenomenon and experiments that involve complicated energy levels, such as $\Lambda$-type levels and N-type levels, elude researchers as it is difficult to realize such energy levels. Recently, a tunable four-level system in circuit QED [PRL 111, 153601 (2013)] has been realized by driving the qubit-cavity system to the nesting regime. We have observed electromagnetically induced transparency (EIT) with all one-photon processes using three energy levels of the four-level system [arXiv:1704.08777]. By incorporating the fourth level of that system and EIT, we can explore a rich physics of quantum optics, including giant cross-phase modulation and wave mixing. In this talk, we will present our recent progress on demonstrating quantum optics experiment with the four-level system in circuit QED.