Frequency-tunable superconducting microwave resonators based on quantum paraelectricity

Frequency tuning enables both high-sensitivity and wide-bandwidth detection in the resonator. Potassium tantalate (KTO) is a quantum paraelectric with tunable permittivity and low microwave loss at cryogenic temperature, making it promising for tunable devices. Here, we demonstrate a superconducting microwave resonator on a KTO substrate. The resonant frequency can be tuned by ∼40 MHz using a 9 V bias at 20 mK, with no observable hysteresis. The absence of hysteresis is a key metric for practical devices, addressing one of the major obstacles in other paraelectric-based tunable resonators. More interestingly, we observed that the quality factor oscillates during voltage tuning, arising from the competition between phonon hardening and charge injection. Therefore, our work provides a practical tunable resonator platform and offers a potential approach to study energy-loss characteristics in quantum paraelectrics.