Low-loss single-photon NbN microwave resonators on Si

Abstract

We present coplanar waveguide (CPW) microwave resonators with exceptionally low loss produced from NbN sputtered on Si. The NbN films are deposited with a modest RF substrate bias during reactive DC magnetron sputtering at a substrate temperature of 250 °C and can achieve a critical temperature as high as 15 K depending on the N2 flow rate. We measure the internal quality factors (Qi) of two such resonators at high-powers near saturation and report high-power quality factors in excess of 1.2 × 106 at 200 mK and 3.5 × 105 at 2 K. We also measure the temperature-dependent frequency shift at high power levels and the quality factor at single-photon power levels. From these measurements, we find a low-power (average photon number less than one) Qi value of 4.2 × 105 at 200 mK, which is consistent with a system limited by two-level-system loss.