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

• Published in: IEEE transactions on applied superconductivity 2014-12, Vol.24 (6), p.1-5
• Main Authors: de Graaf, S. E., Danilov, A. V., Kubatkin, S. E.
• Format: Article
• Language: English
• Subjects: Amplitude modulation; Devices; Dynamic tests; Dynamics; Electron spin resonance (ESR); Frequency modulation; Pound-Drever-Hall (PDH) locking; Q-factor; Quality factor; Real time; Resonant frequency; Resonators; Sensitivity; Sensors; Superconducting microwave devices; superconducting microwave resonators; Superconductivity; Time
• ISSN: 1051-8223; 1558-2515; 1558-2515
• DOI: 10.1109/TASC.2014.2316202

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 ·10 5 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.