Cross-Coupling First-Order Gradient Superconducting Quantum Interference Device for Current Sensing

High sensitivity and low noise of superconducting quantum interference devices make them ideal for reading the minute changes in resistance of a transition-edge sensor, which occurs when it absorbs energy or power. A series of first-order gradient, cross-coupling octagonal SQUIDs specifically tailored for use in TES were developed and fabricated for the advantage of lower parasitic capacitance compared with the overlap-coupling ones. It is obtained that a lower screening parameter and increased shunt resistance per junction lead to a higher flux-to-voltage transfer coefficient. This enhancement significantly boosts detection sensitivity and effectively minimizes noise contributions from electronics operating at room temperature. The low-temperature measurement results of the sample with an input coil of 3.5 turns indicate that a small device current white noise of 4.8 pA/√Hz, a device flux white noise of 1.1 μΦ₀/√Hz, and an optimal flux-to-voltage transfer coefficient of 338.2 μV/Φ₀ are achieved. The bandwidth of a SQUID current sensor with a smaller inductance of the input coil and a larger shunt resistance exceeds 10 MHz. SQUID current sensors, featuring octagonal structures with the first-order gradient cross-coupling, exhibit low flux noise, low current noise, and a high flux-to-voltage transfer coefficient, which can satisfy the requirements of TES applications.