Characterizing Niobium Nitride-Based Superconducting Coplanar Waveguide Resonators for Microwave Hybrid Circuit Quantum Electrodynamics

The high critical magnetic field and relatively high critical temperature of niobium nitride (NbN) make it a promising material for applications in superconducting quantum technology. However, NbN-based superconducting circuits are sensitive to decoherence sources such as two-level system (TLS) defects. Here, we numerically and experimentally investigate NbN superconducting microwave coplanar waveguide resonator arrays with a 100 nm thickness, capacitively coupled to a common coplanar waveguide on a silicon chip. We observe that the resonators' internal quality factor ($Q_{i}$) decreases from $Q_{i}\sim 1.07\times 10^{6}$ in a high power regime to $Q_{i}\sim 1.36\times 10^{5}$ in the single photon regime. Data from this study is consistent with the TLS theory, which describes the TLS interactions in resonator substrates and interfaces. Moreover, we study the temperature ($T$) dependence of $Q_{i}$ and resonance frequency $f_{r}$ of coplanar waveguide resonators to characterize the quasiparticle density. We observe that the increase in kinetic inductance at higher temperatures is the main reason for the frequency shift. Finally, we measure $f_{r}$ and $Q_{i}$ in response to in-plane magnetic fields ($B_{||}$). We verify that $Q_{i}$ stays well above $10^{4}$ up to $B_{||}$ = 240 mT in the photon number $< \! n_{ph} \! >$ = 1.8 at $T$ = 100 mK. Our results could enable the development of robust microwave superconducting circuits for circuit quantum electrodynamics at high magnetic fields required for hybrid superconducting-semiconducting quantum circuits.