Adelaide engineers use junction-free loops to turn SQUID arrays into absolute magnetometers

A Physical Review Applied paper shows bare superconducting loops can mimic the varied loop sizes SQIF sensors need, aiming to keep sensitivity and simplify manufacturing.

Researchers at Adelaide University in South Australia report a simpler way to build arrays of superconducting quantum interference devices (SQUIDs) that can measure absolute magnetic field without giving up the sensitivity that makes them attractive. The work, by PhD students Ross Monaghan and Jonathan Marenkovic and Associate Professor Giuseppe Tettamanzi of the university's Quantum and Nano Technology Group (QuaNTeG), was published in Physical Review Applied on October 26 (volume 26, article 044077), with the full text also on arXiv.

The problem

SQUIDs are loops of superconductor interrupted by Josephson junctions, and they are among the most sensitive magnetic field detectors available. Combining many SQUID loops into a two-dimensional array produces very high-performing radio-frequency sensors. The catch, as the university puts it, is that these devices have struggled to identify when a magnetic field is exactly zero.

The usual fix, used in superconducting quantum interference filters (SQIFs), makes the loop areas deliberately unequal so the array can work as an absolute magnetometer. According to the paper, that requirement conflicts with optimizing each individual loop: changing a loop's size also changes its inductance and a key design parameter that must be tightly controlled for good performance. That compromise keeps such arrays from reaching their full potential, which the authors describe as limited only by the standard quantum limit. The university's announcement notes that existing designs can need hundreds or thousands of differently sized loops.

The technical details

The Adelaide team kept every loop the same size and instead replaced selected loops with "bare" superconducting sections that contain no Josephson junctions. Those bare loops change how magnetic flux is shared across the circuit, so the array behaves as if its loops had different areas. The authors call this a "synthetic area spread" and derive an analytical formula linking the placement of bare loops to the equivalent spread of physical areas.

They then fabricated several 2D SQIF arrays containing bare loops and operated them at very low temperatures. The measured devices behaved in line with the theory, the researchers report.

"By introducing carefully positioned bare loops, we can achieve the same result while preserving the conditions needed for maximum sensitivity," Tettamanzi said.

Why it matters

If uniform loops can do the job, manufacturers would no longer need to fabricate large arrays with many different loop geometries, which the university says could make devices easier and cheaper to build. The university lists brain monitoring, more sensitive radio-frequency receivers, low-noise communications and electromagnetic sensing as possible applications.

What to watch

The university says further testing is needed before the technology can be commercialized. The concept is the subject of an international patent application, and follow-up studies on device performance and new applications are under way. The Quantum Insider carried the university's release on November 2.