Critical fluorescence of a transmon at the Schmid transition

TL;DR

This paper studies how a transmon qubit embedded in a high-impedance circuit exhibits critical fluorescence behavior at the Schmid transition, revealing spectral signatures of the transition point through inelastic photon scattering.

Contribution

It provides the first analysis of fluorescence spectra of a transmon at the Schmid transition, linking spectral features to the circuit impedance and qubit parameters.

Findings

Fluorescence spectrum signals the Schmid transition point.

Quasielastic scattering dominates at high impedance.

Dependence of scattering on circuit and qubit parameters.

Abstract

We investigate inelastic microwave photon scattering by a transmon qubit embedded in a high-impedance circuit. The transmon undergoes a charge-localization (Schmid) transition upon the impedance reaching the critical value. Due to the unique transmon level structure, the fluorescence spectrum carries a signature of the transition point. At higher circuit impedance, quasielastic photon scattering may account for the main part of the inelastic scattering cross-section; we find its dependence on the qubit and circuit parameters.