| We have performed experiments to uncover the source of excess low frequency flux noise in superconducting circuits. We report measurements of the temperature dependence of the quasistatic flux in SQUID circuits demonstrating a high density, ≈ 5 × 1017/m2, of unpaired spins on the superconductor surface. We have also characterized the temperature dependent susceptibility of the surface spins. The susceptibility fluctuates with a 1/f spectrum and the fluctuations are correlated with the flux noise. Finally, we describe experiments that, for the first time, demonstrate a reduction in flux noise magnitude by engineering the metal-dielectric interface. |
