@article {ISI:000484374000020,
title = {Quantum electrodynamics of a superconductor-insulator phase transition},
journal = {Nat. Phys.},
volume = {15},
number = {9},
year = {2019},
month = {SEP},
pages = {930+},
publisher = {NATURE PUBLISHING GROUP},
type = {Article},
abstract = {A chain of Josephson junctions represents one of the simplest many-body models undergoing a superconductor-insulator quantum phase transition(1,2). Apart from zero resistance, the superconducting state is necessarily accompanied by a sound-like mode due to collective oscillations of the phase of the complex-valued order parameter(3,4). Little is known about the fate of this mode on entering the insulating state, where the order parameter{\textquoteright}s amplitude remains non-zero, but the phase ordering is {\textquoteleft}melted{\textquoteright} by quantum fluctuations(5). Here, we show that the phase mode survives far into the insulating regime, such that megaohm-resistance chains can carry gigahertz-frequency alternating currents as nearly ideal superconductors. The insulator reveals itself through interaction-induced broadening and random frequency shifts of collective mode resonances. Our spectroscopic experiment puts forward the problem of quantum electrodynamics of a Bose glass for both theory and experiments(6-8). By pushing the chain parameters deeper into the insulating state, we achieved a wave impedance of the phase mode exceeding the predicted critical value by an order of magnitude(9-14). The effective fine structure constant of such a one-dimensional electromagnetic vacuum exceeds unity, promising transformative applications to quantum science and technology.},
issn = {1745-2473},
doi = {10.1038/s41567-019-0553-1},
author = {Kuzmin, R. and Mencia, R. and Grabon, N. and Mehta, N. and Lin, Y-H and Manucharyan, V. E.}
}