R. G. Herb Condensed Matter Seminars
In a separate experiment, we investigate qubit dephasing (T2) due to photon shot noise in a flux qubit transversally coupled to a coplanar microwave resonator. Due to the AC Stark effect, photon fluctuations in the resonator cause frequency shifts of the qubit, which in turn lead to dephasing. While this is universally understood, we have made the first quantitative spectroscopy of this noise for both thermal (i.e., residual photons from higher temperature stages) and coherent photons (residual photons from the readout and control pulses). By mapping out the noise power spectral density seen by the qubit, we uniquely identify thermal shot noise as the dominant source of dephasing. When implementing the CPMG dynamical-decoupling protocol, we are able mitigate to the adverse influence of the photon shot noise, and improve T2 Echo ~ 40 us to reach T2 CPMG ~ 80 us ~ 2*T1. Furthermore, by improving the filtering for thermal noise in a subsequent cooldown, we are able to reduce the residual photon population to 0.0004, resulting in T2 echo times approaching 100 us .
 Science 354, 1573 (2016)
 Nature Communications 7, 12964 (2016)