Prokof'ev, NikolaiStamp, P.C.E2024-04-262024-04-262000-01-01https://hdl.handle.net/20.500.14394/40549This is the pre-published version harvested from ArXiv.We 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 ô is determined by the linewidth Eo of spin bath states, which can be reduced by a correct choice of ring geometry and isotopic purification. Eo can be measured in either field sweep or microwave absorption experiments, allowing both a test of the theory and design control.Physical Sciences and Mathematicsarticle