Two-Stage Dc-Squid Amplifier Achieves Low Noise with 100 SQUID Cells

Scientists are continually striving to improve the sensitivity of magnetic detectors for applications such as observing the faint signals from the cosmos. Nan Li, Mengjie Song, and Sixiao Hu, from their respective institutions, alongside Wentao Wu, Songqing Liu, Tangchong Kuang et al., have developed a novel low-noise two-stage dc-SQUID amplifier specifically designed for reading out superconducting transition edge sensors (TESs). This research is significant because it addresses a critical need for highly sensitive readout systems in next-generation experiments , notably a cosmic microwave background (CMB) polarization telescope operating in the 22-48GHz range. Their innovative design, featuring a four-cell input SQUID and a 100-cell series SQUID array, demonstrably achieves high signal gain alongside effective noise control, reaching a measured magnetic flux noise of approximately and fulfilling the stringent requirements of TES-based detectors for CMB observation and beyond.

The input SQUID, a double-transformer type with four active cells and two dummy cells, features a 40μm x 12μm washer hole dimension and an inductance of 140 pH, optimised for effective signal coupling. This input stage is paired with a 100-cell SSA, each cell mirroring the input SQUID’s gradiometer design but with slightly modified dimensions of 40μm x 9μm and 3μm x 3μm Josephson junctions. This cascaded architecture, combined with on-chip low-pass filters, dramatically enhances the signal-to-noise ratio, crucial for detecting the subtle variations indicative of weak signals.

This exceptional performance demonstrably satisfies the stringent low-noise requirements for CMB TES detectors and opens avenues for application in a wide range of other TES-based detection systems. The team’s success in achieving such low noise levels is attributed to the careful design of the SQUID cells, the implementation of asymmetric bias injection, and the inclusion of dummy structures to enhance gradiometry. This breakthrough establishes a new benchmark in low-noise readout electronics for TES detectors, paving the way for more sensitive and accurate measurements in diverse fields. The Ali primordial gravitational wave detection project, specifically the AliCPT-40G telescope, will directly benefit from this technology, enabling more precise observation of CMB polarization and a deeper understanding of galactic foregrounds. The developed two-stage dc-SQUID circuit, fabricated using high-quality Nb/AlAlOx/Nb trilayer Josephson junctions, promises to significantly improve the performance of future TES-based experiments in areas such as millimeter wave astronomy, X-ray detection, and the search for dark matter.

Two-Stage dc-SQUID Array for CMB Detection

This innovative architecture allows for exceptionally sensitive magnetic field detection, vital for the telescope’s CMB polarisation measurements. This measurement was conducted using dedicated low-noise instrumentation and careful shielding.

Low-noise two-stage SQUID for TES detectors

Data shows the noise contribution from the SSA, NEISSA, is inversely correlated with the flux conversion coefficient IΦ, meaning an increase in IΦ leads to a reduction in back-end noise. Room-temperature electronic noise was further attenuated by the SSA’s magnetic flux conversion coefficient VΦ, resulting in a lower overall system noise contribution, NEIelec. The team obtained current sensitivities, VΦ and IΦ, by meticulously measuring the magnetic flux response (V-Φ) curves of both SQUIDs. The V-Φ characteristics of the fabricated input SQUID and SSA chips were measured at 300 mK within the ADR system, with each chip bonded to a printed circuit board (PCB) and mounted on a gold-plated copper cold plate.

Scientists recorded current sensitivities of 8μA/Φ0 and 40μA/Φ0 for the input SQUID’s junction loop and input coil respectively, and 27μA/Φ0 and 38μA/Φ0 for the SSA. To enhance system noise performance, the two-stage dc-SQUID was bonded and cooled to 300 mK in the ADR, utilising an ultra-low-noise Magnicon flux-locked loop readout circuit with voltage noise of 0.33 nV/√Hz and current noise of 2.6 pA/√Hz. The SSA demonstrated a maximum voltage swing of approximately 3mV at 23μA current bias, with a maximum flux conversion coefficient Vφ of approximately 10mV/Φ0. The maximum current swing measured through the input SQUID, under a 4 μV voltage bias, was approximately 7μA, constrained by the linear gain interval around the SSA working point.

Tests prove the maximum value of IΦ, at the working point, is approximately 45μA/Φ0. The flat noise power density was measured at 1 μΦ0/√Hz at 1kHz with a 10 kΩ feedback resistor, decreasing to 0.5 μΦ0/√Hz with a 100 kΩ resistor. The system noise equivalent current, NEIsys, was calculated to be about 4 pA/√Hz at 1kHz and 2.4 pA/√Hz at 10kHz, significantly lower than the typical 100 pA/√Hz for TES systems. Finally, the intrinsic noise of the SSA system was measured to be approximately 0.25 μΦ0/√Hz at 10kHz, with a corresponding NEISSA of about 1.2 pA/√Hz.

Low-noise SQUID for CMB polarisation telescopes

This performance satisfies the stringent low-noise requirements for CMB TES calorimeters and is broadly applicable to other TES detector applications. The authors acknowledge that further research is needed to optimise the system for even lower noise performance and to explore its capabilities with different TES designs. Future work could also investigate the integration of this readout system with larger arrays of TES detectors for more extensive CMB observations. This achievement represents a significant advancement in sensitive magnetic detection, enabling more precise measurements of faint signals from astronomical sources. While the current results are promising, the authors note the need for continued optimisation and exploration of the system’s potential with diverse TES configurations, paving the way for improved cosmological observations.