Quasiparticle Dynamics in NbN Superconducting Microwave Resonators at Single Photon Regime

Reliable operation of superconducting quantum circuits demands effective control over quasiparticles, which introduce energy distributions below the superconducting gap and act as a dominant source of decoherence. Here, we investigate the impact of quasiparticle dynamics on niobium nitride (NbN) microwave coplanar waveguide resonators on silicon chips. By performing sub-Kelvin measurements of resonance frequency and internal quality factor across temperature sweeps, we find links between quasiparticle energy and superconducting circuit performance. Calculations of the complex conductivity of the NbN film reveal the quantitative role of quasiparticle density in experimental results. These findings deepen understanding of quasiparticle-induced losses, paving the way toward engineering more resilient superconducting resonators, with broad implications for scalable and fault-tolerant quantum computing architectures.