Thin-film superconducting NbTi microwave resonators for cryogenic thermometry

Abstract: Superconducting microwave resonators have recently gained a primary importance in the development of cryogenic applications, such as circuit quantum electrodynamics, electron spin resonance spectroscopy and particles detection for high-energy physics and astrophysics. In this work, we investigate the influence of the film thickness on the temperature response of microfabricated Nb50Ti50 superconducting resonators. S-shaped split ring resonators (S-SRRs), 20 nm to 150 nm thick, are designed to be electromagnetically coupled with standard Cu coplanar waveguides (CPWs) and their microwave properties are characterized at temperatures below 10 K. The combined contributions of the kinetic inductance LK(T) increase and the decreasing loaded quality factor QL induce an optimum condition on the temperature sensitivity and resolution of the resonators, for thinner films. A noise equivalent temperature (NET) as low as 0.5 uK/Hz^1/2, at 1 Hz, is reported for 100 nm thick resonators at 4.2 K. We also asses the possibility of implementing a multiplexed frequency readout, allowing for the simultaneous temperature tracking of several sensors along a single CPW. Such results demonstrate the possibility to perform a distributed cryogenic temperature monitoring, with a sub-uK resolution. In such a perspective, superconducting S-SRRs, eventually benefiting from an even higher LK(T), might be exploited to accurately monitor the on-chip temperature of devices operating in cryogenic conditions.