Observation of Finite-frequency Superconductivity in Insulating 1D Josephson Junction Chains

A 1D chain of Josephson junctions undergoes a BKT-type superconductor to insulator transition (SIT) driven by quantum fluctuations of the phase of the superconducting order parameter. Away from the transition a superconducting chain experiences two types of fluctuations. The first type corresponds to small vibrations of the phase and gives rise to propagating 1D plasmon waves. The second type corresponds to a slip of the phase by 2π. Theory predicts that phase slips induce the insulating state once the wave impedance of plasmons exceeds the value of R_Q/4 ~ 2 kOhm, where R_Q = 6.5 kOhm is the superconducting resistance quantum. We report microwave spectroscopy of nanofabricated 1D chains of Al/AlOx/Al tunnel junctions, containing 30,000+ junctions. Remarkably, we find that plasmons propagate along the entire chain even when the wave impedance exceeds 20 kOhm, suggesting that the insulating chain behaves as superconductor to finite-frequency currents. This new experimental insight can help to clarify the nature of the insulating phase and the bosonic SIT transition. The high wave impedance of plasmons translates to an effective fine structure constant near unity which opens up a path to studying extreme regimes of light-matter coupling.