On-Chip Microwave Sensing of Nonequilibrium Quasiparticles in α-Tantalum Superconducting Circuits on Silicon for...

The performance and scalability of superconducting quantum circuits are fundamentally constrained by nonequilibrium quasiparticles, which induce microwave losses that limit resonator quality factors and qubit coherence times. Understanding and mitigating these excitations is therefore central to advancing scalable quantum technologies. Here, we demonstrate on-chip microwave sensing of quasiparticles in high-Q α-tantalum coplanar waveguide resonators on silicon, operated in the single-photon regime. Temperature-dependent measurements reveal persistent nonequilibrium quasiparticles at millikelvin temperatures, producing a measurable suppression of the internal quality factor (Q_i) relative to theoretical expectations. By benchmarking across materials, we find that the quasiparticle density in α-Ta is approximately one-third that of NbN at equivalent normalized temperatures (T/T_c), directly correlating with reduced microwave loss. Our methodology establishes a scalable platform for probing quasiparticle dynamics and points toward new routes for engineering superconducting circuits with improved coherence, with impact on qubit readout resonators, kinetic-inductance detectors, and emerging quantum processors and sensors.