Qubit Noise Sensing via Induced Photon Loss in a Superconducting Cavity

TL;DR

This paper introduces a novel method to measure qubit frequency noise by converting it into photon loss in a superconducting cavity, enabling noise characterization at higher frequencies and during driven operation.

Contribution

The authors demonstrate a new protocol that converts qubit frequency noise into photon loss, allowing for enhanced noise spectral analysis beyond traditional qubit-based methods.

Findings

Validated the protocol with injected noise, showing expected scaling.

Placed an upper bound on qubit frequency-noise spectral density at 508 MHz.

Enabled noise measurement during strong driving conditions.

Abstract

Characterizing noise in superconducting qubits is essential for improving coherence and gate performance. Conventional noise-sensing methods typically use the qubit itself as the sensor, which limits both accessible bandwidth and applicability during driven operation. Here, we demonstrate a method for measuring qubit frequency noise by converting it into photon loss in a coupled high-Q superconducting cavity. We use repeated mid-circuit qubit measurements with post-selection to separate this induced loss from intrinsic cavity decay. We validate the protocol using injected noise and show that the extracted loss scales as expected with the applied noise strength. Without added noise, we place an upper bound of $5\times10^3\,\mathrm{Hz}^2/\,\mathrm{Hz}$ on the qubit frequency-noise power spectral density at 508 MHz. The protocol opens access to a higher-frequency spectral window than standard qubit-based spectroscopy and may enable noise characterization during strong driving.