Highly linear proximity-based double-loop SQUID operating above 4 K

We demonstrate a highly linear superconducting quantum interference device (SQUID) interferometer based on a double-loop (Bi-SQUID) architecture incorporating three superconductor-normal metal-superconductor ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>S</a:mi> </a:math> - <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>N</c:mi> </c:math> - <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>S</e:mi> </e:math> ) junctions. Fabricated using niobium-gold technology, the device exhibits robust operation at liquid helium temperatures, with a superconducting transition temperature of 8.5 K. The dc flux-to-voltage transfer function demonstrates sharp, symmetric, and highly linear behavior at temperatures up to 5 K. Bi-SQUIDs featuring our single-element <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>S</g:mi> </g:math> - <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>N</i:mi> </i:math> - <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>S</k:mi> </k:math> design represent an interesting and original approach to this field, as they exhibit a numerically estimated spurious-free dynamic range linearity exceeding 60 dB, achieved in a single element, simplifying the requirements in terms of arrays containing hundreds of junctions. These results highlight the potential of proximity-based Bi-SQUIDs for compact and highly linear cryogenic interferometric devices, motivating future investigations of their dynamic and noise performance for applied implementations such as quantum sensing, magnetometry, and biomedical diagnostics.