Quantum electrodynamics of superconductor-insulator transitions in Josephson junction chains.

It is customary to associate a superconducting phase with zero resistance and an insulating phase with infinite resistance. However, experimentally measured resistance often does not attain either value, and quantum phase transition physics is hidden in the scaling of resistance with temperature, size, magnetic field, etc. Instead, we consider the finite frequency spectroscopy of the collective phase-mode at excitation levels far below a single quantum. In this case, the superconducting phase manifests itself through a non-zero phase-stiffness, while the insulating phase results in the damping of the phase-mode by quantum phase-slip fluctuations. The damping of the phase-mode is independent of the system’s size, but scales with the frequency, which is a more reliable scaling parameter than temperature. We present the recent results of our spectroscopic studies on three notable SIT systems: a high-inductance ring interrupted by a weak junction (quasicharge localization) [1]; (ii) a homogeneous chain of Josephson junctions (1D superfluid to Bose-glass insulator transition) [2]; and (iii) a chain of relatively strong junctions containing one weak junction impurity (Shmid-Bulgadaev dissipative transition).

[1] I. V. Pechenezhskiy et al. “The superconducting quasicharge qubit” Nature 585, 368 (2020).
[2] R. Kuzmin et al. “Quantum electrodynamics of a superconductor–insulator phase transition” Nature Physics 15, 930 (2019).