Two-dimensional Superconducting Quantum Interference Arrays Overcome Area Limitations for Enhanced Magnetometer Performance

Superconducting Quantum Interference Devices, or SQUIDs, underpin many advanced sensing technologies, and recent advances demonstrate that arranging these devices into two-dimensional arrays significantly enhances their performance as radio frequency sensors. Ross D. Monaghan, Jonathan L. Marenkovic, and Giuseppe C. Tettamanzi, all from The University of Adelaide, now reveal a method to overcome a key limitation in these arrays, the need for varying the physical area of each SQUID loop to function as precise absolute magnetometers. The team demonstrates that strategically incorporating sections of superconducting circuit without Josephson junctions creates what they term a ‘synthetic area spread’, effectively mimicking the performance of physically varied loops and unlocking the full potential of these arrays. This breakthrough, supported by both theoretical modelling and experimental verification through fabricated 2D arrays, paves the way for the development of even more sensitive and accurate absolute sensors with broader applications.

This research focuses on achieving absolute magnetometer functionality, where the array accurately measures magnetic field strength, without the complexity of fabricating physically distinct loop areas. The team hypothesizes that these bare loops create synthetic loop areas, influencing the magnetic flux response of the array and achieving comparable results to more complex designs without increasing fabrication difficulty. The researchers combined theoretical modeling with experimental validation, utilizing Voltage-Magnetic Flux (VMF) curves to characterize the array’s response to magnetic fields.

They fabricated arrays with and without bare loops, carefully controlling the geometry and materials to ensure accurate comparisons. The results demonstrate that bare loops can be used as a design tool to tune the sensitivity and response of SQUID arrays, offering a simpler and potentially more cost-effective alternative to traditional methods. Detailed analysis of the fabrication process reveals that the circuits are constructed from niobium, a widely used superconducting material, incorporating niobium/aluminum-oxide/niobium Josephson junctions, essential components for SQUID functionality. The multi-layer fabrication process achieves high reproducibility, with approximately 80% of fabricated devices operating correctly. Traditional designs require physical separation between SQUID loops to achieve optimal performance, restricting design flexibility and sensor density. This work demonstrates that strategically inserting bare superconducting loops, sections of circuit without Josephson junctions, into the array allows it to function as an absolute magnetometer even without physical area spread. The team developed a complete analytical formulation linking the distribution of these bare loops to what they term a “synthetic area spread”, effectively mimicking the performance of physically separated loops.

To experimentally verify this theory, the researchers fabricated 16×16 arrays of SQUID loops, some incorporating rows of bare loops between lines of SQUIDs and others constructed without them. Each unit cell consisted of SQUID loops and, in some designs, bare superconducting loops, with the area of each SQUID loop twice that of the bare loop. A homogenous magnetic field was applied to each array using an external superconducting coil, and the voltage response was measured by applying a bias current. The experimental setup mirrored theoretical conditions, allowing for direct comparison between predicted and observed behaviour.

The resulting Voltage-Magnetic Flux (VMF) curves revealed a striking difference between the two array designs. Arrays containing bare loops exhibited a significant anti-peak in the VMF response, indicating the emergence of synthetic loop areas induced by the bare loops. Conversely, arrays without bare loops showed no such anti-peak. These findings confirm that the careful addition of bare loops can dramatically modify the VMF response, enabling the fabrication of high-performance absolute sensors without the need for physical area spread. Traditionally, achieving absolute magnetometer functionality demanded that each loop in the array possess a unique area, limiting performance potential. This work overcomes this limitation by selectively inserting ‘bare’ superconducting loops, loops containing no Josephson junctions, into the 2D array architecture. The team developed a complete analytical formulation that establishes a one-to-one correspondence between the distribution of these bare loops and what they term a ‘synthetic area spread’.

This synthetic spread effectively mimics the behaviour of physically separated loops, allowing the array to achieve the desired absolute Voltage-Magnetic Flux response without requiring actual area differences. Experiments involved fabricating several 2D SQUID arrays incorporating these bare superconducting loops, and the results align precisely with the theoretical predictions. Measurements demonstrate that a 3×2 array with equal loop areas, when combined with the synthetic area approach, produces a Voltage-Magnetic Flux response that closely matches the expected behaviour. Further experiments with a 5×2 array containing two rows of solely bare loops confirm this alignment. Analysis of a 10×10 array, incorporating either one or three rows of bare loops, reveals minimal differences between the bare loop configuration and the synthetic area approach, demonstrating the robustness of the method. The team’s work opens the way to a broader use of this technology for the fabrication of ultra-high-performance absolute sensors, offering a new pathway for sensitive magnetic field detection.

Synthetic Area Spread Enables Absolute Magnetometry

Researchers have demonstrated a new approach to designing two-dimensional superconducting quantum interference device (SQUID) arrays, enabling them to function as highly sensitive absolute magnetometers without compromising performance. This work introduces the concept of ‘synthetic area spread’, achieved by strategically incorporating sections of superconducting material lacking Josephson junctions into the array design. The team developed a complete analytical formulation that establishes a direct relationship between the distribution of these bare loops and the equivalent physical spread of traditional, incommensurate SQUID loops.

This synthetic spread allows the arrays to operate as absolute magnetometers while maintaining the optimized performance of each individual SQUID loop, potentially unlocking the ability to reach the quantum limit of detection for electromagnetic signals. Experimental verification, through the fabrication and testing of several 2D SQUID arrays, confirms the validity of the theoretical model. The authors acknowledge that the performance of these arrays is dependent on precise fabrication and control of the bare loop geometry. Future research will likely focus on optimizing these parameters and exploring the potential of this technology in various sensing applications. The team has filed a patent application detailing the concept of synthetic area spread and its implementation in mixed SQUID arrays, suggesting a pathway towards practical implementation of this technology.