Localization of Metal-Induced Gap States at the Metal-Insulator Interface:Origin of Flux Noise in SQUIDs and Superconducting Qubits

TL;DR

This paper explains the origin of flux noise in superconducting devices by showing that disorder at the metal-insulator interface creates localized states with magnetic moments, accounting for observed noise levels.

Contribution

It demonstrates that potential disorder at the metal-insulator interface induces localized gap states with magnetic moments, explaining flux noise in superconducting devices.

Findings

Localized gap states produce magnetic moments.

Disorder levels match observed areal density.

Flux noise arises from these localized magnetic moments.

Abstract

The origin of magnetic flux noise in Superconducting Quantum Interference Devices with a power spectrum scaling as $1/f$ ($f$ is frequency) has been a puzzle for over 20 years. This noise limits the decoherence time of superconducting qubits. A consensus has emerged that the noise arises from fluctuating spins of localized electrons with an areal density of $5\times10^{17}$m$^{-2}$. We show that, in the presence of potential disorder at the metal-insulator interface, some of the metal-induced gap states become localized and produce local moments. A modest level of disorder yields the observed areal density.