Flux noise in SQUIDs: Evidence for temperature dependent spin-diffusion
The intrinsic flux noise observed in superconducting quantum interference devices (SQUIDs) is thought to be due to the fluctuation of electron spin impurities, but the frequency and temperature dependence observed in experiments does not agree with the usual 1/f models. We present theoretical calculations and experimental measurements of flux noise in rf SQUID flux qubits that shows how these observations can be interpreted in terms of a spin-diffusion constant that increases with temperature. Our measurements of flux noise in the 20-80 mK temperature range allows the extraction of the spin-diffusion constant and its temperature dependence, suggesting that a phase transition takes place at T ~ 1 mK.