1/f Flux Noise from Surface Magnetic Defects

Robert McDermott

Madison, Wisconsin

Low-frequency 1/f flux noise is a dominant source of dephasing in Josephson quantum bits. Work by our group and others has revealed the presence of a high density of unpaired spins at the surfaces of superconducting thin films; it is now believed that these spins are the source of the noise, although the microscopic noise mechanism is not understood. In this talk, I will describe SQUID-based measurements of surface spin magnetization and susceptibility that suggest the existence of correlated clusters of fluctuating spins. In addition, I will discuss x-ray absorption measurements that point to O-containing adsorbates as a dominant source of surface magnetism. I will discuss ongoing efforts to reduce the surface spin density and suppress noise by suitable surface treatments.