University of Maryland
Quantum electrodynamics of a 1D superconductor near superconductor-insulator transition
We explore superconductor-insulator transition in fabricated ultralong 1D chains of Josephson tunnel junctions (30,000+ junctions) using microwave spectroscopy. Classically, such a chain can be viewed as a "telegraph" transmission line where vacuum permeability is replaced by the Josephson kinetic inductance. Quantum fluctuations of the condensate, in theory, give rise to a superconductor-insulator transition of the Kosterlitz-Thouless class. Here we report that our chains, surprisingly, support propagating microwaves with low damping even when the measured chain parameters are deep in the insulating phase. The main feature of the observed radiation is that it is slower than light in vacuum by a factor of over 300, presumably due to the large kinetic inductance. In electrical engineer's language, this translates to a wave impedance of the transmission line in the 10's of kOhms range. One can equivalently interpret the slow "light" as if the effective fine structure constant for our media exceeds unity. We speculate that our observation may be explained by a finite-energy picture of the quantum phase transition. We also discuss how our chains can be utilized to explore ultra-strong interactions of light and matter at the level of single quanta and simulate dynamics of non-linear Luttinger liquids with impurities.