  The origin of magnetic flux noise in Superconducting Quantum Interference
Devices with a power spectrum scaling as $1/f$ ($f$ is frequency) has been a
puzzle for over 20 years. This noise limits the decoherence time of
superconducting qubits. A consensus has emerged that the noise arises from
fluctuating spins of localized electrons with an areal density of
$5\times10^{17}$m$^{-2}$. We show that, in the presence of potential disorder
at the metal-insulator interface, some of the metal-induced gap states become
localized and produce local moments. A modest level of disorder yields the
observed areal density.
