Slow noise processes in superconducting resonators

Slow noise processes, with characteristic timescales $\sim$1 s, have been studied in planar superconducting resonators. A frequency-locked loop is employed to track deviations of the resonator center frequency with high precision and bandwidth. Comparative measurements are made in varying microwave drive and temperature, and between bare resonators and those with an additional dielectric layer. All resonators are found to exhibit flicker frequency noise which increases with decreasing microwave drive. We also show that an increase in temperature results in a saturation of flicker noise in resonators with an additional dielectric layer, while bare resonators stop exhibiting flicker noise, instead showing a random frequency walk process.

Figure 1

Left: Photograph of device, consisting of a long meandering inductive part and an inter-digitated capacitor. For covered resonators the capacitive part is coated by the additional dielectric layer (red strip). Right: Scatter plot comparing the loss tangents of bare resonators with those covered by ${\text{Al}}_2{\text{O}}_3$ and ${\text{HfO}}_2$. Plot highlights density of TLFs to be uniform with frequency and for covered resonators to have a loss tangent between 5–10 times higher than the bare resonator. Circled is a bare resonator, B1, at 6.93 GHz and an ${\text{Al}}_2{\text{O}}_3$ covered resonator, A1, at 5.08 GHz, which are featured in later measurements.

Figure 2

The rms frequency deviation as a function of measurement integration time. Successive shapes correspond to different microwave drive powers: triangles $=$ $-$85 dBm, circles $=$ $-$95 dBm, and squares $=$ $-$105 dBm. Plots (a) and (b) are of a bare resonator, B1, and an aluminium oxide covered resonator, A1, in plots (c) and (d), where the temperature is 90 mK for plots (a) and (c) and 700 mK for plots (b) and (d).