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 (S-N-S) 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 S-N-S 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.