Quantum electrodynamics of a superconductor-insulator phase transition
Abstract: A chain of Josephson junctions implements one of the simplest many-body models undergoing a superconductor-insulator quantum phase transition between states with zero and infinite resistance. Apart from zero resistance, the superconducting state is necessarily accompanied by a sound-like phase mode due to collective oscillations of the phase of the complex-valued order parameter. Exciting the phase mode results in transverse photons propagating along the chain. Surprisingly little is known about the fate of this mode upon entering the insulating state, where the order parameter's amplitude remains non-zero, but the phase ordering is “melted” by quantum fluctuations. I will present momentum-resolved radio-frequency spectroscopy of collective modes in nanofabricated chains of Al/AlOx/Al tunnel junctions. Our key finding is that the phase mode survives remarkably far into the insulating regime, such that chains with a MOhm-range DC resistance carry AC currents as nearly ideal superconductors. At GHz-frequencies, the insulator reveals itself by an intrinsic decoherence of collective modes and a scatter of their eigenfrequencies, originating from coupling to quantum phase slips. Deep in an insulating state we observed waves propagation with the speed of light down to 8*10^5 m/s and the wave impedance up to 23 kOhm, with latter quantity exceeding predicted insulator transition values by an order of magnitude. Quite generally, the existence of an extended phase mode in a bosonic insulator realizes a 1D quantum electrodynamics with the fine structure constant exceeding a unity, promising potentially transformative applications in quantum physics and technology.
Notes: Lunch served at 12:00 pm, talk at 12:15 pm.