Nitride-based superconducting microwave coplanar waveguide resonators with internal quality factors above one million for circuit quantum electrodynamics

Niobium nitride (NbN) is a promising candidate for applications in superconducting quantum technology because of its high critical magnetic field and relatively high critical temperature. In contrast, NbN-based devices are more susceptible to decoherence sources such as two-level system defects. The purpose of this experiment is to investigate superconducting microwave coplanar waveguide resonators based on NbN on a silicon substrate at millikelvin temperatures from a single photon number regime to high power regime in the presence of in-plane magnetic fields.  

We fabricated an array of superconducting NbN microwave resonators, with a thickness of 100 nm, that are capacitively coupled to a single common feedline. According to this study, the measured loss in the resonator at a millikelvin temperature and a single photon regime has a good agreement with TLS loss theory model. We observe that the resonators’ internal quality factor (Qi) decreases from Qi in a high photon number to Qi in a single-photon regime at a base temperature of T = 26.6 mK. Moreover, we study Qi and frequency tuning of the coplanar waveguide resonators as a function of temperature to characterise the quasi-particle (qp) density of NbN. In our experiment, we observe that the primary reason for the frequency shift, occurring at higher temperatures is an increase in kinetic inductance which happens due to splitting cooper pairs at this temperature range. Finally, we extract resonance frequency and Qi in response to in-plane magnetic fields (B || ). We verify that Qi stays well above up to B || =120 mT in the single-photon regime.