Quantum electrodynamics of a superconductor-insulator quantum phase transition II: experiment

We report on our experimental probing of the superconductor-insulator quantum phase transition (QPT) in a 1D Josephson chain in the high-frequency and short-wavelength limit [1]. By contrast, traditional experiments focus on finding a universal scaling of resistance with temperature and other system parameters. The quantum BKT transition resisted this approach for two decades. In part I, we overview the possible issues with the DC transport measurements and describe the advantages of viewing a QPT from the high-energy side. In part II, we show the results of our microwave spectroscopy of long chains of Al/AlOx/Al tunnel junctions. First, we find that an insulating chain can carry AC currents (in a GHz-band) as a near-perfect superconductor. Second, deviations from perfect superconductivity can be resolved. These deviations appear as frequency-dependent linewidths of the collective modes and as a scattering of their frequencies. Both effects arise from interactions and constitute the high-frequency footprint of the insulating phase near the critical point. Our new approach can help bridge theory with the experiments in chains, thin films, nanowires, and cold atom realizations of 1D systems.
[1]R. Kuzmin, R. Mencia, N. Grabon, N. Mehta, Y.-H. Lin, V. E. Manucharyan, arXiv 1805.07379