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Infrared, Millimeter-Wave, and Terahertz Technologies

Development of second-order gradient DC SQUIDs for the readout of superconducting TES detectors

Z. F. Feng, Xuefeng Zhou, Q. X. Ma, Wen Zhang, Peng Li, Z. Wang, Jiaqiang Zhong, W. Miao, Yuan Ren, Qi Yao, V. P. Koshelets, L. Fillppenko, M. Yu. Fominsky, J. Li, S. C. Shi

Published Nov 19, 2025• 0 citations
Publisher

Abstract

Superconducting transition-edge sensors (TESs) are highly sensitive detectors, can detect electromagnetic wave radiations from millimeter/submillimeter, optical to x/γ rays, suitable for astrophysics, quantum information, and biosensing. Direct current superconducting quantum interference devices (DC SQUIDs) are the detector of choice for TES readout, thanks to their low noise, low impedance, and low power dissipation. In this work, we designed the DC SQUID with a second-order gradient structure, which can effectively mitigate the effect of external magnetic flux. We optimized the fabrication process and successfully fabricated the high-performance DC SQUID with Nb/Al-AlOx/Nb Josephson junctions (critical current density: JC~1μA/μm2). The properties of the fabricated DC SQUID were in good agreement with the simulation results. Meanwhile, the measured flux noise was 6 μΦ0/√Hz, corresponding to a current noise of 100 pA/√Hz. Furthermore, we demonstrated the capability of the DC SQUID to read out the TES detector, including I-V curves, photon pulse response, and noise performance.

DOI
10.1117/12.3073803

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Superconducting transition-edge sensors (TESs) are highly sensitive detectors, can detect electromagnetic wave radiations from millimeter/submillimeter, optical to x/γ rays, suitable for astrophysics, quantum information, and biosensing. Direct current superconducting quantum interference devices (DC SQUIDs) are the detector of choice for TES readout, thanks to their low noise, low impedance, and low power dissipation. In this work, we designed the DC SQUID with a second-order gradient structure, which can effectively mitigate the effect of external magnetic flux. We optimized the fabrication process and successfully fabricated the high-performance DC SQUID with Nb/Al-AlOx/Nb Josephson junctions (critical current density: JC~1μA/μm2). The properties of the fabricated DC SQUID were in good agreement with the simulation results. Meanwhile, the measured flux noise was 6 μΦ0/√Hz, corresponding to a current noise of 100 pA/√Hz. Furthermore, we demonstrated the capability of the DC SQUID to read out the TES detector, including I-V curves, photon pulse response, and noise performance.
Abstract Direct-current superconducting quantum interference devices (dc-SQUIDs) are one of the most sensitive magnetic detectors. These sensors are extensively used in the readout of superconducting transition edge sensors (TESs), which are used for the detection of weak signals. A cosmic microwave background (CMB) polarization telescope operating in 22–48 GHz is currently under developing. The TESs calorimeter of the telescope will be readout by a time-division multiplexer SQUID readout system. We develop a two-stage dc-SQUID amplifier circuit, comprising an input-stage SQUID with 4 SQUID cells and a series SQUID array with 100 SQUID cells. This configuration has been shown to achieve extremely high signal gain while effectively controlling system noise. We assess the system noise at 300 mK in an adiabatic demagnetization refrigerator. The measured magnetic flux noise of the two-stage SQUID circuit system is approximately 0.3 μ Φ 0 / Hz at 10 kHz. The current noise equivalent to the input coil of input SQUID is about 2.4 pA / Hz . This result meets the low-noise readout requirements of the CMB TES and other applications with TES detectors.
Superconducting transition-edge sensors (TESs) are highly sensitive detectors and can detect electromagnetic wave radiations from millimeter/submillimeter, optical to 𝑥/γrays, suitable for cosmology, astrophysics, quantum information, and biosensing. In principle, thousands of TESs even more are required to enhance the detection efficiency for large-scale survey. Among other multiplexing schemes, microwave SQUID multiplexer (μMUX), consisting of resonators and RF SQUIDs, has a bandwidth of several GHz, thus multiplexing factor on the order of thousands, more suitable for readout of large TES arrays. We designed and fabricated superconducting coplanar waveguide (CPW) resonators with a high qualityfactor and second-order gradient RF SQUID with two inductive coupling structures respectively. Then, we optimized the critical current density of the Josephson junction and measured the mutual inductance parameters of the second-order gradient structure SQUID, which are consistent with the simulation results. Finally, we fabricated a cryogenic μMUX chip based on RF SQUID and resonator. We discussed the results of the development of μMUX in more detail.
Direct-current superconducting quantum interference devices (dc-SQUIDs) are one of the most sensitive magnetic detectors. These sensors are extensively used in the readout of superconducting transition edge sensors (TESs), which are used for the detection of weak signals. A cosmic microwave background (CMB) polarization telescope operating in 22-48 GHz is currently under developing. The TESs calorimeter of the telescope will be readout by a time-division multiplexer (TDM) SQUID readout system. We develop a two-stage dc-SQUID amplifier circuit, comprising an input-stage SQUID with 4 SQUID cells and a series SQUID array (SSA) with 100 SQUID cells. This configuration has been shown to achieve extremely high signal gain while effectively controlling system noise. We assess the system noise at 300300300 mKmKmK in an adiabatic demagnetisation refrigerator (ADR). The the measured magnetic flux noise of the two-stage SQUID circuit system is approximately 0.30.30.3 μΦ0/HzμΦ_{0}/\sqrt{Hz}μΦ0​/Hz​ at 101010 kHzkHzkHz. The current noise equivalent to the input coil of input SQUID is about 2.42.42.4 pA/HzpA/\sqrt{Hz}pA/Hz​. This result meets the low-noise readout requirements of the CMB TES and other applications with TES detectors.
Superconducting transition-edge sensors (TESs) with high sensitivity can detect electromagnetic wave radiations from millimeter/submillimeter, optical to x/γ rays, and have been successfully used in many fields. To improve detection efficiency and sensitivity, large-scale TES arrays are required in combination with multiplexing readout electronics. Compared to other multiplexing schemes, microwave SQUID multiplexing (μMUX), consisting of high-quality microwave superconducting resonators and RF SQUIDs, is most suitable since it provides a wider bandwidth (GHz) and consequently higher multiplexing factor. We designed two types of circuits with different mutual inductances (MT) between the resonator and the RF SQUID: 2.3 pH for the weak-coupling channel and 6.5 pH for the strong- coupling channel. The resonator and RF SQUID washers made of 150 nm Nb are first fabricated by magnetron-sputtering and reactive ion etching (RIE), followed by deposition of Nb/Al-AlOx/Nb trilayer and definition of the Josephson junctions of the RF SQUIDs. This work presents a μMUX demonstration chip with five channels with Q1values of 787 and 3626, and the measured frequency shifts (Δfpp) of ∼500 kHz and 4 MHz for the weak- and strong-coupling channels, respectively. The open-loop equivalent noise current, obtained from the measured phase noise and resonator responsivity, is ∼80 pA/ √Hz with the strong-coupling channel.
A low-noise superconducting quantum interference device (dc-SQUID) with simplified design and readout electronics is highly desirable for various applications. We demonstrate that introducing an asymmetric shunt resistor significantly enhances the noise performance while maintaining the inherent simplicity and tunability of the SQUID design. By optimizing the asymmetry factor, our asymmetric shunted second-order gradient SQUID current sensor reaches a low white flux noise of 0.65 μΦ0/√Hz using a direct readout strategy, representing a threefold improvement over its symmetric counterpart. This noise level surpasses that of the SQUID with the additional positive feedback method and is comparable to the series SQUID arrays, making the asymmetric shunted SQUID suitable for meeting the readout requirements of various scientific instruments.
A cold preamplifier based on superconducting quantum interference devices (SQUIDs) is currently the preferred readout technology for the low-noise transition edge sensor (TES). In this work, we have designed and fabricated a series SQUID array (SSA) amplifier for the TES detector readout circuit. In this SSA amplifier, each SQUID cell is composed of a first-order gradiometer formed using two equally large square washers, and an on-chip low pass filter (LPF) as a radio-frequency (RF) choke has been developed to reduce the Josephson oscillation interference between individual SQUID cells. In addition, a highly symmetric layout has been designed carefully to provide a fully consistent embedded electromagnetic environment and achieve coherent flux operation. The measured results show smooth V–Φ characteristics and a swing voltage that increases linearly with increasing SQUID cell number N. A white flux noise level as low as 0.28 μ Φ 0/Hz1/2 is achieved at 0.1 K, corresponding to a low current noise level of 7 pA/Hz1/2. We analyze the measured noise contribution at mK-scale temperatures and find that the dominant noise derives from a combination of the SSA intrinsic noise and the equivalent current noise of the room temperature electronics.