New multi-SQUID device cuts costs, improves sensitivity

Physicists in the United Kingdom have built a multi-SQUID (superconducting quantum interference device) that can operate at 77K, the boiling point of liquid nitrogen. According to research published in Applied Physics Letters, the new device outperforms the industry standard niobium/aluminum trilayer single-SQUIDs maintained at 4.2K. “Since our SQUID arrays operate at 77K using liquid nitrogen, they provide a magnetic sensor that is very cost effective, considering that operation of SQUIDs at 4.2K requires the use of liquid helium-4, which is much more expensive and also requires extensive training for its handling,” says Boris Chesca, a physicist at Loughborough University and first author on the new paper.

A SQUID can detect minuscule magnetic fields and is useful in applications ranging from medical imaging of soft tissue to oil prospecting. SQUIDs work by converting a measure of magnetic intensity called flux into a voltage. Low temperature SQUIDs are inherently more sensitive, but Chesca and his colleagues theorized that a large array of high temperature SQUIDs could achieve the same performance as a single low temperature one. When a series of SQUIDs is strung together the voltage output scales in direct proportion to the number of SQUIDs, while the noise in the measurement only increases as a square root of the SQUID count. More SQUIDs, thus, result in stronger signals and more sensitive measurements.

The approach, however, only works if all the SQUIDs experience the same magnetic flux (a condition called flux coherency) and the interaction between each SQUID and its neighbors is minimized. Both are tricky conditions to meet because of parasitic fluxes that form during normal SQUID operation. “Flux coherency could never be reached in the previous approaches in large SQUID arrays with more than 30 SQUIDs,” Chesca says.

The key difference in the array designed by the team from Loughborough University and Nottingham University has to do with a feature called a flux focuser. Flux focusers are large areas of superconductor that improve magnetic field sensitivity and minimize parasitic fluxes. In previous SQUID arrays there were either no flux focusers or the flux focusers were so large that they limited the number of SQUIDs that could be integrated onto a single chip. In the new design, the flux focusers are narrow, but long. By keeping the width of the focusers identical to the SQUIDs’ width, the researchers could string together a long series of SQUIDs while still achieving a high degree of flux coherence and field sensitivity by increasing the length of the flux focusers.

Chesca and his colleagues tested the performance of the arrays (one 770 SQUIDs long and the other 484 SQUIDs long) and compared them to the performance of optimized low temperature single-SQUIDs operating at 4.2K, generally the gold standard for industrial magnetometers. The team found the new arrays’ white flux noise, a key performance parameter linked to sensitivity, were superior to those of the 4.2K SQUIDs.

The researchers are currently working on optimizing the shape and design of the SQUID arrays to maximize magnetic field sensitivity. “The most exciting aspect of our result,” says Chesca, “is that replacing single SQUIDs operating at 4.2K with SQUID arrays operating at 77K would be, for the first time, not only a cheaper and more user-friendly solution, but a solution that comes with no compromise regarding noise resolution.”