Spectral measurement of the thermal excitation of a superconducting qubit

TL;DR

This paper measures the thermal excitation of a superconducting qubit via noise spectrum analysis, revealing an unexpected non-radiative relaxation channel that affects qubit energy relaxation rates.

Contribution

It introduces a spectral measurement method to analyze thermal excitation and relaxation mechanisms in superconducting qubits, uncovering non-radiative energy loss channels.

Findings

Qubit excitation correlates with thermal radiation levels.

Qubit relaxation rate decreases with temperature.

Evidence of non-radiative relaxation channels.

Abstract

We report the measurement of the fluctuations of a transmon qubit through the noise spectrum of the microwave signal that measures its state. The amplitude of the Lorentzian noise power spectrum allows to determine the average qubit excitation, in agreement with the estimated thermal radiation reaching the sample. Its width yields the qubit energy relaxation rate which decreases with temperature, contrary to the predictions for a two-level system solely coupled to thermal radiation. This indicates the existence of another non-radiative energy relaxation channel for the qubit.