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 (Qi) decreases from Qi∼1.07×106 in a high power regime to Qi∼1.36×105 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 Qi and resonance frequency fr 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 fr and Qi in the single photon regime in response to in-plane magnetic fields (B||). We verify that Qi stays well above 104 up to B|| = 240 mT in the photon number <nph> = 1.8 at T = 100 mK. Our results could enable the development of robust microwave superconducting circuits for circuit quantum electrodynamics (cQED) at high magnetic fields required for hybrid superconducting-semiconducting quantum circuits.