Revealing spin-flip two-level systems using ultra-thin film superconducting resonators

TL;DR

This paper uncovers spin-related behaviors of two-level systems in ultra-thin TiN superconducting resonators, revealing how defect spins influence device noise and loss, and proposing a spin-flip TLS model involving spin-orbit coupling.

Contribution

It introduces a novel spin-flip TLS model that explains anomalous frequency shifts in superconducting resonators, advancing understanding of spin-dependent noise in quantum devices.

Findings

Observed increase in resonant frequency at low magnetic fields.

Proposed a spin-flip TLS model with spin-orbit coupling.

Reproduced frequency-field relationship and temperature dependence.

Abstract

Material disorders are one of the major sources of noise and loss in solid-state quantum devices, whose behaviors are often modeled as two-level systems (TLSs) formed by charge tunneling between neighboring sites. However, the role of their spins in tunneling and its impact on device performance remain highly unexplored. In this work, employing ultra-thin TiN superconducting resonators, we reveal anomalous TLS behaviors by demonstrating an unexpected increase in resonant frequency at low magnetic fields. Furthermore, a spin-flip TLS model is proposed, in which an effective spin-orbit coupling is generated by inhomogeneous local magnetic fields from defect spins. This mechanism mixes charge tunnelings and spin flips, quantitatively reproducing the observed frequency-field relationship and its temperature dependence. This work deepens the understanding of spin-dependent TLS behaviors, offering the possibility of magnetically engineering noise and loss in solid-state quantum devices.