Hydrogen as a Source of Flux Noise in SQUIDs

TL;DR

This paper investigates hydrogen atoms as a source of flux noise in superconducting qubits, using density functional theory to analyze their magnetic properties and proposing graphene coating as a mitigation strategy.

Contribution

It identifies hydrogen atoms on a-Al2O3 surfaces as significant spin sources and suggests a graphene coating to prevent flux noise in SQUIDs.

Findings

Hydrogen atoms have sizeable spin moments on a-Al2O3 surfaces.

H adatoms attract gas molecules like O2, creating additional spin sources.

Graphene coating can eliminate H-induced surface spins.

Abstract

Superconducting qubits are hampered by flux noise produced by surface spins from a variety of microscopic sources. Recent experiments indicated that hydrogen (H) atoms may be one of those sources. Using density functional theory calculations, we report that H atoms either embedded in, or adsorbed on, an a-Al2O3(0001) surface have sizeable spin moments ranging from 0.81 to 0.87 uB with energy barriers for spin reorientation as low as ~10 mK. Furthermore, H adatoms on the surface attract gas molecules such as O2, producing new spin sources. We propose coating the surface with graphene to eliminate H-induced surface spins and to protect the surface from other adsorbates.