Candidate source of flux noise in SQUIDs: adsorbed oxygen molecules

TL;DR

This paper suggests that adsorbed oxygen molecules on surfaces could be the source of flux noise in SQUIDs, with theoretical and simulation evidence supporting their magnetic contribution.

Contribution

It introduces a novel hypothesis that O2 molecules cause flux noise in SQUIDs, supported by density functional theory and Monte Carlo simulations.

Findings

O2 molecules on surfaces have magnetic moments of ~1.8 μB.

Spin orientation affects the energy barrier for spin rotations (~10 mK).

Simulated magnetization noise exhibits 1/f behavior consistent with experimental flux noise.

Abstract

A major obstacle to using SQUIDs as qubits is flux noise. We propose that the heretofore mysterious spins producing flux noise could be $O_2$ molecules adsorbed on the surface. Using density functional theory calculations, we find that an $O_2$ molecule adsorbed on an {\alpha}-alumina surface has a magnetic moment of ~1.8 {\mu}B. When the spin is oriented perpendicular to the axis of the O-O bond, the barrier to spin rotations is about 10 mK. Monte Carlo simulations of ferromagnetically coupled, anisotropic XY spins on a square lattice find 1/f magnetization noise, consistent with flux noise in Al SQUIDs.