Accurate Real-Time Monitoring of Quality Factor and Center Frequency of Superconducting Resonators

In this paper, we track both center frequency and quality factor of a superconducting microwave resonator with very high precision using a modified Pound-Drever-Hall frequency locking scheme. The quality factor measurements can be done with high enough bandwidth to study different processes in a superconducting resonator in real time. With this added functionality, the frequency readout remains unaffected, with typical accuracy around 10-30 Hz/√(Hz), whereas the cavity loss rate can be determined to within 0.1% on a millisecond time scale (34 dB/√(Hz)). As we demonstrate, this can be particularly useful for electron spin resonance measurements, where we achieve a minimum number of detectable spins coupled to the resonator N = 5 ·10⁵ spins/√(Hz), based on quality factor measurements. It also enables the study of fast dynamic processes, for example, vortex dynamics, in these devices on a short time scale.