Evaluation of Flux Trapping in Superconducting Circuits

At ASC 2006 we presented our technique for experimental evaluation and comparative study of the damage created by flux trapping in superconductor circuits. Here, we present results of this study for different designs of functionally identical shift registers. The only differences between the designs are sizes, shapes and densities of moats surrounding the circuitry. The measured critical magnetic fields are inversely proportional to the second power of the distance between the moat and the protected area. This observation is qualitatively correct even for non-uniform patterns of straight and bent moats. With better understanding of the protection mechanism we were able to increase the threshold magnetic fields for our circuits from 4 mG to about 20 mG. Our design recommendations allow the magnetic shield requirements to be dramatically relaxed. In particular, we predict that with a 5 micrometer distance between moats, well designed circuits would be able to operate in Earth field (0.3 G to 0.6 G) without any shielding.