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.