Deep strong coupling in a circuit QED system (1) - Introduction -*

Recently, light-matter interaction at the single-photon level has been demonstrated in superconducting circuits (circuit-QED). The interaction energy between a superconducting artificial atom and an excitation quantum of a harmonic oscillator in the microwave region has been shown to be very large, at least a few thousand times that of the atom-photon interaction obtained using Rydberg atoms [1]. It is also intriguing that, depending on the circuit design, the relevant physical parameters of this system can be controlled at will. In particular, an interaction energy as large as the transition energy of a superconducting artificial atom or a harmonic oscillator is possible, where totally new states, such as a spontaneously generated Schrödinger-cat-like correlated ground state of light and matter, have been predicted [2,3]. In this talk, I will introduce the motivation and the significance of the research, methods to achieve such a strong interaction, and a brief overview of the obtained results. [1] J. Johansson, S. Saito, T. Meno, H. Nakano, M. Ueda, K. Semba, and H. Takayanagi, Phys. Rev. Lett. 96, 127006 (2006). [2] S. Ashhab and Franco Nori, Phys. Rev. A 81, 042311 (2010). [3] S. Ashhab Phys. Rev. A 87, 013826 (2013).