Studies on the Characterization and Measurement Optimization of Superconducting Microwave Resonators

Abstract

This work presents a study aimed at improving the accuracy and efficiency of low-temperature loss-tangent measurements in superconducting resonators. Measurements were performed on aluminum and niobium devices, where a plateau in the internal quality factor at the single-photon level was observed, consistent with prior reports. By truncating the acquired resonance data, it was shown that the loss tangent experienced no systematic shift even when only four points spanned the resonance linewidth; the resulting increase in uncertainty was attributed to reduced effective averaging. Based on this result, an optimized data acquisition scheme was developed, reducing measurement time by a factor of four while maintaining approximately 1% accuracy. Further improvements were achieved through the use of a lower-noise HEMT amplifier, which reduced measurement noise and decreased acquisition time to 60% of the original. Additional circuit modifications showed that improved infrared shielding reduced total resonator loss, while the nonlinear behavior at high RF power was attributed to intrinsic device nonlinearity rather than external circuitry. Finally, crossover temperature measurements showed agreement with BCS theory at high temperatures, although its accuracy could be limited by not fully saturated TLS-loss, indicating the need for improved device designs.