Circuit-QED-based investigations of two-dimensional Josephson junction arrays in the quantum regime*

Josephson junction arrays (JJAs) offer model systems for studying various quantum many-body phenomena. One of the remarkable phenomena in JJAs is the quantum phase transition between superconducting and insulating phases, which occurs due to the competition between the Josephson energy E_J and the charging energy E_C. Here we study dynamics of two-dimensional JJAs in the quantum critical regime (E_J ~ E_C) using a circuit-QED approach. We find that the internal loss of the cavity at the zero-temperature limit increases steeply as approaching the quantum critical point. Furthermore, unlike in the classical superconducting regime (E_J >> E_C) where the cavity loss exhibits a peak at transition temperature T_c due to dissipative motion of free vortices generated by the Berezinskii-Kosterlitz-Thouless mechanism [1], we do not observe a peak in the cavity loss at T_c in the quantum critical regime. These observations suggest that the loss mechanism is different from that in the classical superconducting regime. In the talk, we will discuss the results in connection with the quantum phase transition.
[1] R. Cosmic, et al., Phys. Rev. B 102, 094509 (2020).