Circuit-QED Studies of Josephson Junction Arrays in the Quantum Regime*

Josephson junction arrays (JJAs) in superconducting circuits offer model systems for studying various many-body phenomena both in the classical and quantum regimes. One of the remarkable phenomena observed in JJAs is the quantum phase transition between superconducting and insulating phases associated with the competition between the Josephson energy and the charging energy. In our previous study, we used a circuit-QED technique to study dynamics of the superconductor-metal transition of a classical JJA at a single photon level and found that the internal loss of the cavity shows a peak at the transition temperature. Here we use the experimental technique to explore JJAs in the quantum regime. We find that the temperature at which a peak appears in the cavity loss decreases when the charging energy becomes more dominant than the Josephson energy, i.e., the quantum effect becomes more significant. In the talk, we will discuss the result in connection with the quantum phase transition between the superconducting and insulating phases.