2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/16437We consider a SQUID tunneling between 2 nearly degenerate flux states. Decoherence caused by paramagnetic and nuclear spins in the low-$T$ limit is shown to be much stronger than that from electronic excitations. The decoherence time $τ_ϕ$ is determined by the linewidth $E_o$ of spin bath states, which can be reduced by a correct choice of ring geometry and isotopic purification. $E_o$ can be measured in either field sweep or microwave absorption experiments, allowing both a test of the theory and design control.5 pages, LaTex, 3 figures, submitted to PRL in May, 2000Condensed Mattertext