Adelaide Engineers Fix a Flaw in SQUID Quantum Sensors With 'Bare Loops'

Quantum engineers at Adelaide University say they have found a simpler way to improve some of the most sensitive electromagnetic field detectors ever built.

The research, published in Physical Review Applied, was led by the university's Quantum and Nano Technology Group and addresses a long-standing limit on superconducting quantum interference devices, known as SQUIDs. The team says the advance could help brain imaging, medical diagnostics, communications technology and next-generation quantum devices.

SQUIDs are among the most sensitive magnetic field detectors created. They are used in specialized applications including brain imaging systems, scientific instruments and advanced radio-frequency sensors. But while they can detect incredibly small changes in a magnetic field, they have struggled to accurately identify when the field is exactly zero.

Until now, engineers have worked around the problem by building circuits with hundreds or even thousands of superconducting loops of different sizes. The approach works, but it reduces the overall sensitivity of the device.

The Adelaide team, which includes doctoral students Ross Monaghan and Jonathan Marenkovic, found a simpler route. They kept the loops the same size and replaced some of them with "bare loops" that contain no Josephson junctions, the key component of traditional SQUIDs. The bare loops change how magnetic signals move through the circuit and create what the researchers call a "synthetic area spread." The sensor then behaves as if it contained many differently sized loops, without needing them.

Lead researcher Giuseppe Tettamanzi, an associate professor in the university's School of Chemical Engineering, said the discovery could help unlock the full potential of quantum sensing. "For decades, scientists have known how to make these devices identify an absolute magnetic field, but doing so required design compromises that reduced their performance," he said. "Our work shows there is another way. By introducing carefully positioned bare loops, we can achieve the same result while preserving the conditions needed for maximum sensitivity."

The researchers first developed the theory and then tested it on fabricated superconducting circuits operating at very low temperatures. The results showed the new design behaved exactly as predicted.

Tettamanzi said the advance could eventually benefit a wide range of fields. "Highly sensitive superconducting quantum sensors have enormous potential in medicine, communications, industry and defense," he said. "The closer we get to the fundamental quantum limit of detection, the more information we can extract from very weak signals." Possible uses include better brain monitoring systems, more sensitive radio-frequency receivers, low-noise communication technologies and advanced electromagnetic sensing.

The approach could also make future devices cheaper and easier to build, because it removes the need to fabricate large arrays of loops in many sizes. The team says more testing is needed before the technology can be commercialized. The concept is the subject of an international patent application, and further studies are already underway to improve performance and explore new applications.