Distinguishing coherent and thermal photon noise in a circuit QED system

Understand

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.