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IEEE Transactions on Applied Superconductivity

Development of Microwave SQUID Multiplexer for Readout of Transition-Edge Sensor Array

Zhi-Fa Feng, Xianfeng Zhou, Wen Zhang, Zheng Wang, Qingxiao Ma, Peizhan Li, Jiaqiang Zhong, Wei MIAO, Yuan Ren, Qi-Jun Yao, V. P. Koshelets, Lyudmila V. Filippenko, M. Yu. Fominsky, Jing Li, Shengcai SHI

Published Jan 14, 2026• 2 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2026.3653239

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IEEE Transactions on Applied Superconductivity•Jan 1, 2013•28 citations
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.
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.
Abstract A microwave SQUID multiplexer ( μ MUX) with a high multiplexing factor is a promising readout approach for large-scale superconducting transition-edge sensor (TES) bolometer arrays. This paper reports on the characteristics of 16-channel μ MUX prototype chips and a 60-channel readout module developed to read out TES bolometers for the Ali cosmic microwave background polarization telescope (AliCPT). We designed two different coupling structures between the resonator and the radio-frequency superconducting interference device: design A with a mutual inductance ( M T ) of 2.3 pH and design B with an M T of 6.5 pH. For design A, the mean resonator bandwidth ( Δ f BW ) and peak-to-peak frequency shift ( Δ f pp ) are approximately 610 kHz and 290 kHz, respectively. The measured mean open-loop current noise is less than 40 pA / Hz . Preliminary results indicate that crosstalk between adjacent channels is below 1% in our design A μ MUX chip. A measured channel with design B shows a Δ f BW of 586 kHz and a current noise of 33 pA / Hz , similar to the design A channels. In comparison, the Δ f pp of 803 kHz is much larger than the design A channels due to a larger M T . The readout module, consisting of two design A chips and two design B chips, shows 60 resonances, corresponding to a yield of 94%. In the future, we will place 32 design A channels within the same chip dimension to increase the channel density and build a larger readout module suitable for AliCPT TES arrays.
Dissipationless microwave radio frequency superconducting interference device (RF-SQUID) multiplexers are attractive for readouts of large format arrays of transition edge sensors (TES). This multiplexer has the potential for readout bandwidth of several GHz and power consumption of ≈0.1 pW per pixel, which are ≈103 and ≈10-4 times as large as those of DC-SQUID-based conventional multiplexers. To evaluate their readout performance for TES photon counters ranging from near infrared to gamma ray, white noise of this multiplexer was experimentally studied at 4 K. The reason for this was, firstly, to avoid the low-frequency fluctuation obvious at around 0.1 K but trivial for the energy resolution of photon counters, and secondly for a feasibility study of readout operation at 4 K for extended applications. To increase resonant Q at 4 K and maintain low-noise SQUID operation, multiplexer chips consisting of niobium nitride (NbN)-based coplanar-waveguide resonators and niobium (Nb)-based RF-SQUIDs have been developed. This hybrid multiplexer exhibited 1×104≤QI≤2×104 and the square root of spectral density of current noise referred to the SQUID input √SI =31 pA/√Hz, where QI is the internal Q of resonators. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Systematic noise measurement with various microwave readout power PMR and two-directional readout on the complex plane of transmission component of scattering matrix S21 makes the contribution of each noise source obvious. By decreasing these noises to the degree achievable by current technology, we predicted the microwave RF-SQUID multiplexer would exhibit √SI ≤5 pA/√Hz; i.e., close to √SI of state-of-the-art DC-SQUID-based multiplexers.
Transition Edge Sensors (TESs) are highly sensitive cryogenic detectors widely used in astronomy, high-energy physics and nuclear radiation detection. As TES is increasingly deployed in large-scale arrays to improve sensitivity and effective collecting area, multiplexed readout becomes essential for reducing system complexity. Microwave SQUID multiplexing (μMUX) technology combining superconducting resonators with radio-frequency superconducting quantum interference devices (RF-SQUIDs), represents a state-of-the-art solution. In this work, a room-temperature readout prototype for TES μMUX readout was designed. The system features a scalable two-stage architecture, including a highly integrated, chassis-independent front-end signal processing module and back-end event-reconstruction module. A high-frequency-resolution frequency-comb signal generation method with real-time, per-tone frequency and phase tunability was developed. In addition, optimization of the firmware implementation effectively overcomes the computational-resource bottleneck of the Field Programmable Gate Array (FPGA). The prototype supports autonomous frequency sweeping, parameter diagnostics, and online event reconstruction. Self-test shows S 21 phase noise better than −110 dBc/Hz at −30 dBm, remaining stable with increasing channel count. Joint tests with a μMUX cryogenic system successfully captured the RF-SQUID response and achieved an equivalent noise current of 44 pA/√Hz referred to the TES, validating the prototype as a practical solution of the room-temperature system for TES μMUX readout.
In this article, we have developed a scalable superconducting quantum interference device (SQUID)-based microwave multiplexer (µMUX) readout architecture adapted for arrays of transition edge sensors (TESs) to be exploited in a terahertz security camera. The camera system combines a scanning optics together with a 128-pixel TES array and aims for security gate operations with a standoff detection distance of up to 25 m. The developed frequency-domain µMUX is used to read out all feed-horn-coupled TESs based on aluminum thermistors, with noise limited by intrinsic TES noise. Both the TESs and µMUXs are operated in a compact cryostat with a base temperature of about 0.9 K. The µMUX has been fabricated in the cross-typeNb/AlO_x/Nb\text{Nb}/ \text{AlO}\_{\rm{x}}/ \text{Nb}Nb/AlO_x/NbJosephson junction process developed at the Leibniz Institute of Photonic Technology and incorporates high-quality superconducting thin-film resonators with resonant frequencies in the range of 5–6 GHz. An accordingly implemented field-programmable-gate-array-based readout electronics enables the simultaneous and continuous real-time readout of 128 rf-SQUIDs, including a flux ramp modulation scheme. In operation with the aluminum TES, we achieved a dark noise equivalent power of about2.5  fW/Hz1/22.5\; \text{fW}/ \text{Hz}^{1/ 2}2.5fW/Hz1/2while providing a TES readout rate of 3.75 kHz, necessary for a video frame rate of 25 Hz of the security camera.
Resonant frequencies and SQUID inductancesLSin a microwave SQUID multiplexer are studied for the purpose of reliable design applicable for a future large-format array of transition edge sensors (TESs). The key element of the multiplexer chip is a quarter-wave-length resonator terminated by a dissipationless rf SQUID connected to TES. For the resonator, we found that a small correction to the effective length of the resonators made the designed resonant frequencies fit the experimental one over the whole readout band of 4-8 GHz covered by our cryogenic amplifier. We also found that microstrip-type SQUIDs with vertical loops are suitable for accurate design of small inductanceLS<; 10<; pH to operate the rf SQUID in dissipationless regime. Based on these well-designed resonators and SQUIDs, we have successfully demonstrated the multiplexing of three pulsed signals with 1-ms width and spacing given at the input ports of three SQUIDs. The equivalent noise current refer to the SQUID input is not dominated by SQUIDs and estimated to be 200-270 pA/ √Hz, whose improvement approach is quantitatively discussed.