Quantum electrodynamics of a superconductor-insulator phase transition

Understand

A chain of Josephson junctions implements one of the simplest many-body models undergoing a superconductor-insulator (SI) quantum phase transition between states with zero and infinite resistance.

• 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.
• Exciting this 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.