# On-Chip Microwave Sensing of Nonequilibrium Quasiparticles in α‑Tantalum Superconducting Circuits on Silicon for Scalable Quantum Technologies

## TL;DR

This paper introduces a method to detect and study quasiparticles in superconducting circuits, which could improve the performance of quantum technologies.

## Contribution

The novel contribution is on-chip microwave sensing of nonequilibrium quasiparticles in α-tantalum circuits on silicon.

## Key findings

- Quasiparticle density in α-Ta is about one-third that of NbN at equivalent temperatures.
- Quasiparticles cause measurable suppression of resonator quality factors at millikelvin temperatures.
- The methodology provides a scalable platform for probing quasiparticle dynamics.

## Abstract

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.