A SQUID is a superconducting ring that can measure incredibly small magnetic fields. The best performing ones – that is, the ones exhibiting the best signal-to-noise ratio – have traditionally been those cooled to temperatures of liquid helium, 4.2 K. Handling liquid helium isn't easy, however, and helium itself is becoming increasingly scarce, and increasingly pricey.

Loughborough University's Boris Chesca and colleagues' answer to this problem is a compound SQUID composed of at least 480 individual SQUIDS. Each of these operates at temperatures above 77 K provided by liquid nitrogen, a relatively safe and abundant resource. The reason for the physicists' approach is that the signal – the voltage output – of a SQUID array rises linearly with the number N of individual SQUIDs, while the noise scales only as N½.

SQUID arrays with N in the many thousands have been demonstrated at low, liquid-helium temperatures, but high, liquid-nitrogen temperature versions have been more difficult to realize. One problem is so-called flux coherency, in that all of the component SQUIDs have to experience the same magnetic flux as their neighbours. Another problem is unwanted interactions between the component SQUIDs, created from magnetic fluxes generated by the SQUIDs themselves.

To solve the flux-coherency problem, Chesca and colleagues created their array out of identical SQUIDS on small, 10 mm2 substrates, coated with the common superconductor yttrium barium copper oxide and patterned with 1 µm resolution photolithography. To stymie the interactions, the physicists used devices known as flux focusers to connect only those SQUIDS they wanted to.

The group found that their high-temperature SQUID array containing 770 individual SQUIDs had a white flux noise less than half that of a single SQUID operating at low, liquid-helium temperatures. Combined with the other advantages of high-temperature superconductors, such as their low cost and ability to minimize distance to measured subjects, says Chesca, this development ought to make the new SQUID array very attractive to those who rely on sensitive magnetometers.

"These advantages are particularly significant in magnetoencephalography," said Chesca. "Replacing single-SQUIDs operating at 4.2 K with SQUID arrays operating at 77 K would be, for the first time, not only a cheaper and more user friendly solution but a solution that will also provide superior sensitivity in various magnetic imaging devices."

In a separate press statement, Chesca added: "We believe there should be an immediate interest in the entire SQUID community for the potential replacement of their existing superconducting magnetic sensors based on single SQUIDs operating at 4.2 K with our SQUID arrays operating at 77 K".

Related stories

• Ultralow-field MRI comes out of the lab
• CUBRIC: a focus on brain research
• MEG spots post-traumatic stress disorder
• MEG and MRI: getting it together?