# Distinguishing coherent and thermal photon noise in a circuit QED system. (arXiv:1801.00467v1 [quant-ph])

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.