At millikelvin temperatures and low frequencies f, superconducting flux qubits and SQUIDs both exhibit intrinsic magnetic flux noise. In SQUIDs, the noise power spectrum scales as 1/f^{beta} where beta varies from approximately 0.6 to 1. The amplitude of the 1/f noise ranges from about 1lyxmathsym{textendash}10muPhi_{0}Hz^{-1/2} at 1Hz, although the area of the SQUDs varies over four orders of magnitude. There appears to be no dependence of the noise magnitude on the nature of the superconducting material or the substrate. A local model based on the random flipping of surface electrons between up and down states is consistent with the observed dependence of the noise on area. This model requires an areal electron density of about 5cdot10^{17} m^{-2} to account for the observed magnitude of the noise. Experiments at Wisconsin and Stanford on the paramagnetic susceptibility of SQUIDs and normal metal rings yield the same areal density. Recently, we have developed a model in which the noise is generated by localized states at the superconductor-insulator interface. Detailed calculations show that a modest level of interfacial disorder causes Anderson localization of the metal-induced gap states; an areal density of 5cdot10^{17} m^{-2} is readily achieved. This result suggests that epitaxially grown films with reduced disorder may lead to lower levels of flux noise. Recent studies of the dependence of the slope beta on the geometry of SQUIDs are presented. This work is in collaboration with Steve Anton, Jeff Birenbaum, SangKook Choi, Andrew Fefferman, Sean O’Kelly, Dunghai Lee, Steven Louie, Keenan Pepper, Cristian Urbina and Fred Wellstood, and supported by IARPA and ARO.
Department of Physics, University of California Berkeley