A symmetrical three-junction superconducting quantum interference device

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Abstract
A new symmetrical 1:2:1 split-fed Superconducting Quantum Interference Device (SQUID) has been designed and experimentally investigated. By laying out the control line symmetrically about the center axis of the device and by moving the Josephson junctions in the device out from underneath the control line, we were able to eliminate virtually all the sources of asymmetry in the path of the control line. This asymmetry causes degradation of the threshold curve of the SQUID over a few periods and renders the conventional three-junction SQUID unsuitable for some applications, such as one Josephson analog-to-digital conversion scheme. By eliminating the sources of asymmetry, we were able to make SQUIDs that showed at least eight almost identical periodic lobes. An additional advantage of the symmetrical SQUID is that the insulating SiO layer which defines the junction windows could be used to provide an insulation between the base electrode and the control line. Thus, the control line was defined at the same time as the counterelectrode. This made it possible to make a symmetrical three-junction SQUID with three layers fewer than the planar-type three junction SQUIDs reported previously.

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