Observation of the Schmid-Bulgadaev dissipative quantum phase transition

TL;DR

This paper experimentally observes the Schmid-Bulgadaev dissipative quantum phase transition in a Josephson junction by analyzing its interaction with a transmission line resistor, revealing a switch from superconducting to insulating behavior at a critical resistance.

Contribution

First experimental demonstration of the resistor-induced quantum phase transition in a Josephson junction using electromagnetic scattering measurements.

Findings

Junction scatters electromagnetic excitations as inductance or capacitance depending on impedance

At the phase boundary, the junction acts as an ideal resistor with spontaneous photon down-conversion

The transition is marked by a frequency-independent down-conversion probability

Abstract

Although quantum mechanics applies to many macroscopic superconducting devices, one basic prediction remained controversial for decades. Namely, a Josephson junction connected to a resistor must undergo a dissipation-induced quantum phase transition from superconductor to insulator once the resistor's value exceeds $h/4e^2 \approx 6.5~\textrm{k}\Omega$ ($h$ is Planck's constant, $e$ is the electron charge). Here we finally demonstrate this transition by observing the resistor's internal dynamics. Implementing our resistor as a long transmission line section, we find that a junction scatters electromagnetic excitations in the line as either inductance (superconductor) or capacitance (insulator), depending solely on the line's wave impedance. At the phase boundary, the junction itself acts as ideal resistance: in addition to elastic scattering, incident photons can spontaneously down-convert with a frequency-independent probability, which provides a novel marker of quantum-critical behavior.