Emergent quantum phase transition of a Josephson junction coupled to a high-impedance multimode resonator

TL;DR

This paper investigates how a Josephson junction coupled to a high-impedance multimode resonator undergoes an emergent quantum phase transition, revealing a level anticrossing and universal spectral signatures in the thermodynamic limit.

Contribution

It demonstrates how multimode environments influence the Josephson junction's energies and uncovers a novel phase transition characterized by a level anticrossing involving excited states.

Findings

The transition involves a level anticrossing of the first excited state.

The charging energy dominates when impedance exceeds the resistance quantum.

Spectral signatures of the phase transition match recent experimental observations.

Abstract

The physics of a single Josephson junction coupled to a resistive environment is a long-standing fundamental problem at the center of an intense debate, strongly revived by the advent of superconducting platforms with high-impedance multimode resonators. Here we investigate the emergent criticality of a junction coupled to a multimode resonator when the number of modes is increased. We demonstrate how the multimode environment renormalizes the Josephson and capacitive energies of the junction so that in the thermodynamic limit the charging energy dominates when the impedance is larger than the resistance quantum and is negligible otherwise, independently from the bare ratio between the two energy scales and the compact or extended nature of the phase of the junction. Via exact diagonalization, we find that the transition surprisingly stems from a level anticrossing involving not the ground state, but the first excited state, whose energy gap vanishes in the thermodynamic limit. We clarify the nature of the two phases by pointing at a different behaviour of the ground and excited states and we show that at the transition point the spectrum displays universality not only at low frequencies. In agreement with recent experiments, we reveal striking spectral signatures of the phase transition.