Distinguishing coherent and thermal photon noise in a circuit QED system

TL;DR

This paper presents a method to distinguish and identify the sources of photon noise in a circuit QED system, revealing thermal photons as the main dephasing factor and demonstrating noise suppression techniques.

Contribution

The authors introduce a noise-spectral reconstruction method to differentiate coherent and thermal photons, enabling targeted noise mitigation in circuit QED systems.

Findings

Thermal photons are the primary source of qubit dephasing.

Improved cryogenic attenuation reduces residual thermal photons.

Achieved $T_1$-limited spin-echo decay time.

Abstract

In the cavity-QED architecture, photon number fluctuations from residual cavity photons cause qubit dephasing due to the AC Stark effect. These unwanted photons originate from a variety of sources, such as thermal radiation, leftover measurement photons, and crosstalk. Using a capacitively-shunted flux qubit coupled to a transmission line cavity, we demonstrate a method that identifies and distinguishes coherent and thermal photons based on noise-spectral reconstruction from time-domain spin-locking relaxometry. Using these measurements, we attribute the limiting dephasing source in our system to thermal photons, rather than coherent photons. By improving the cryogenic attenuation on lines leading to the cavity, we successfully suppress residual thermal photons and achieve $T_1$-limited spin-echo decay time. The spin-locking noise spectroscopy technique can readily be applied to other qubit modalities for identifying general asymmetric non-classical noise spectra.