{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"aca0c65a9142f60ce78bfdc0073a597b12e931a058e8ea78f84565ca76835eff","created":"2026-10-03T07:18:00Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"54a9d65f41544b24b4ed2cfe391fb69ff74405850ebb01f936cf649d83c2faf2","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"cm.one-over-f-noise.flux-noise-spins","kind":"well-posed","literature_status":"partially-resolved","n":"1","parents":[],"plain":"Superconducting loops show slow magnetic-flux fluctuations of nearly the same size whatever the loop size or material. Unpaired electron spins on surfaces are blamed, but what they are and how they produce a 1/f spectrum is not settled.","posed_since":"1987","precise":"Low-frequency flux noise S_Phi(1 Hz)^(1/2) ~ 1-10 micro-Phi_0/Hz^(1/2) ($\\Phi_0=hc/2e$), weakly dependent on loop area, is attributed to surface spins of density $\\sim 5e13\\,\\mathrm{cm}^{-2}$ (Koch, DiVincenzo and Clarke 2007). Identify the spin species (adsorbed O2, hydrogen, dangling bonds) and the dynamics producing $1/f$ (spin clusters, spin-glass dynamics, spin diffusion), consistent with the measured temperature dependence and the correlation between flux noise and inductance noise.","problem_ref":null,"references":"","settled_by":"Surface treatments that remove a candidate species and reduce flux noise by the predicted amount, together with a spin model reproducing spectrum, temperature dependence and cross-correlations.","status_note":"Experiments since 2016 link part of the noise to adsorbed molecular oxygen; the spin dynamics producing 1/f remain disputed.","title":"Microscopic origin of 1/f flux noise in SQUIDs and 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