Quasiparticle spectroscopy in tantalum films with different Ta/sapphire interfaces

TL;DR

This study uses quasiparticle spectroscopy via a frequency-domain resonator to investigate low-energy excitations in tantalum films, revealing insights into loss mechanisms affecting superconducting quantum devices.

Contribution

It introduces a non-destructive quasiparticle spectroscopy method tailored for thin films, providing microscopic insights into dissipation sources in superconducting circuits.

Findings

Detection of low-energy excitations linked to two-level systems and Yu-Shiba-Rusinov states.

Correlation between internal quality factors and the presence of subgap states.

Validation of the spectroscopy technique as a tool for quantum materials analysis.

Abstract

One of the crucial aspects of current research in quantum information science is the identification and control of loss mechanisms in superconducting circuits. Although microwave measurements directly quantify device performance, additional techniques that probe quasiparticle excitations in superconducting films are needed to understand the microscopic mechanisms underlying dissipation and decoherence. Here, we present results from quasiparticle spectroscopy of Ta/sapphire films by measuring the Meissner-state magnetic susceptibility using a precision frequency-domain resonator specifically designed for thin films. We find direct evidence for additional low-energy excitations in samples with lower internal quality factors. These excitations are consistent with deep subgap states due to two-level systems, Yu-Shiba-Rusinov states near the gap edge, and perhaps other pair-breaking mechanisms. The developed non-destructive frequency-domain quasiparticle spectroscopy is a valuable addition to the quantum materials toolbox.