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arXiv:2509.07671 · IEEE Transactions on Applied Superconductivity

Full-Scale Microwave SQUID Multiplexer Readout System for Magnetic Microcalorimeters

M. Neidig, Timo Muscheid, R. Gartmann, Luis Eduardo Ardila-Perez, Mathias Wegner, Oliver Sander, Sebastian Kempf

Published May 22, 2026• 0 citations
arXivHTMLPDF

Abstract

The deployment of large cryogenic detector arrays, comprising hundreds to thousands of individual detectors, is highly beneficial for various cutting-edge applications, requiring large statistics, angular resolution or imaging capabilities. The readout of such arrays, however, presents a major challenge in terms of system complexity, parasitic heat load, and cost, which can be overcome only through multiplexing. Among the various multiplexing approaches, microwave SQUID multiplexing ($\mathrm{\mu }$MUXing) currently represents the state of the art, in particular for magnetic microcalorimeter (MMC) readout. In this work, we demonstrate the successful operation of the latest generation of our microwave SQUID multiplexer-based readout system, based on a SQUID multiplexer tailored for MMC readout and a custom full-scale software-defined radio (SDR) electronics, capable of handling up to 400 channels. We operate 14 readout channels simultaneously and demonstrate that the system operates reliably across the entire 4-8 GHz frequency band and achieves low flux white noise levels of$(1.4\pm 0.2)\,\mathrm{\upmu\Phi _{0}/\sqrt{Hz}}$in flux-ramp-demodulated readout. While technical steps regarding the SDR output power, as well as a demonstration of the simultaneous readout of the full 400 channels, remain, our results confirm that our system is fully functional and provides a scalable path towards future large-scale, high-resolution MMC experiments.

DOI
10.1109/tasc.2026.3696057

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arXiv:2509.23569•IEEE Transactions on Applied Superconductivity•Mar 31, 2026•0 citations

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Abstract The readout system with a high multiplexing ratio has become a bottleneck limiting the application of large-scale Transition Edge Sensor (TES) detector arrays. In recent years, the microwave superconducting quantum interference device (SQUID) multiplexer has emerged as a key technology for effectively reading large-scale cryogenic detector arrays. Currently, the microwave SQUID multiplexer is being adopted by an increasing number of experiments due to its capability of achieving a multiplexing ratio of 2000:1 within the readout bandwidth. In this study, we developed a 32-channel microwave SQUID multiplexer prototype. And we measured 8 channels of the prototype. The measured equivalent noise current of the prototype reached 154 pA/√ Hz.

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Abstract The Microwave SQUID Multiplexer ($\mu$MUX) is a critical readout technology for large-scale Superconducting Transition-Edge Sensor (TES) array. It is considered as a promising readout system for both bolometer and calorimeter applications of large-scale TES array. It has been employed in several Cosmic Microwave Background (CMB) experiments, in which the TESes are used as bolometers. However, in the field of calorimeter applications of TESes, such as X-ray detection, the $\mu$MUX system has not been widely used. The main reason is the slew rate of calorimeter pulse signal is much faster than that of bolometer, which will increase the difficulties of data processing in $\mu$MUX electronics. In a typical microwave SQUID Multiplexed readout scheme, the sampling rate is determined by the frequency of the flux-ramp modulation. For CMB experiment, the frequency of the flux-ramp is usually set to about $\mathcal{O}$(10~kHz). However, for X-ray detection, it is necessary to improve the frequency of flux-ramp to about 1~MHz. It is a challenge to implement the high speed data processing with current $\mu$MUX electronics.%The $\mu$Mux readout algorithm is a crucial component.Currently, the implementation of the $\mu$Mux readout algorithm is complex, and the sampling rate for each TES is low, only reaching kHz.We propose a recursive algorithm to reconstruct the fast TES signals with high sampling rate in case of no hardware changes. This algorithm is simple enough to implement in real time data collection. %In this paper, we describe and demonstrate this technique with a reasonable simulation model of $\mu$MUX. The simulation results shows that the recovered sampling rate can break through the frequency limitation of the flux ramp, reaching up to analog-to-digital converter (ADC) sampling rate. The non-linearity is lower to 0.25\% in full dynamic range.

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We report very-low-noise, fast-response, middle-scale multiplexing in a microwave superconducting quantum interference device multiplexer (MW-Mux) as a transition-edge sensor (TES) readout. Our MW-Mux is able to read 40 channels with 500 kHz sampling and has a low readout noise of 0.9 μΦ0/Hz (where Φ0 is the magnetic flux quantum), equivalent to 9 pA/Hz. By contrast, a multiplexer of less than 10 pixels with 500 kHz sampling and ∼2 μΦ0/Hz readout noise has so far been reported in the literature. Owing to the 500 kHz sampling, our MW-Mux exhibits a fast response to detect a TES pulse with a rise time around 12 μs. We demonstrated simultaneous readout of 38 pixels from an array of x-ray TES microcalorimeters. The measured full-width values at half-maximum spectral resolution ranged from 2.79 to 4.56 eV, with a median value of 3.30 eV at 5.9 keV, including a ∼10% contribution of readout noise, i.e., 0.9–1.7 eV.

Applied Physics Letters•Sep 21, 2020•26 citations
The deployment of large cryogenic detector arrays, comprising hundreds to thousands of individual detectors, is highly beneficial for various cutting-edge applications, requiring large statistics, angular resolution or imaging capabilities. The readout of such arrays, however, presents a major challenge in terms of system complexity, parasitic heat load, and cost, which can be overcome only through multiplexing. Among the various multiplexing approaches, microwave SQUID multiplexing (μ\mathrm{\mu }μMUXing) currently represents the state of the art, in particular for magnetic microcalorimeter (MMC) readout. In this work, we demonstrate the successful operation of the latest generation of our microwave SQUID multiplexer-based readout system, based on a SQUID multiplexer tailored for MMC readout and a custom full-scale software-defined radio (SDR) electronics, capable of handling up to 400 channels. We operate 14 readout channels simultaneously and demonstrate that the system operates reliably across the entire 4-8 GHz frequency band and achieves low flux white noise levels of(1.4±0.2) \upmuΦ0/Hz(1.4\pm 0.2)\,\mathrm{\upmu\Phi _{0}/\sqrt{Hz}}(1.4±0.2)\upmuΦ0​/Hz​in flux-ramp-demodulated readout. While technical steps regarding the SDR output power, as well as a demonstration of the simultaneous readout of the full 400 channels, remain, our results confirm that our system is fully functional and provides a scalable path towards future large-scale, high-resolution MMC experiments.
In this work, we report the experimental readout results of aµµµMUX device using a Direct-RF Software-Defined Radio (SDR) prototype based on the ZCU216 Radio-Frequency System-on-Chip (RFSoC) evaluation board. First, the Direct-RF SDR architecture is described. Then, the analog performance of the SDR system was characterized both in loopback and when coupled to the cryogenic multiplexing system to evaluate the noise contributions of the SDR readout system. Later, the SDR system performance was optimized forµµµMUX readout, focusing on bolometric applications. Finally, the minimum demodulated flux noise was obtained through the optimally conditioned readout of aµµµMUX channel. The results obtained showed a minimum demodulated white flux noise of≈4μΦ0/Hz\approx 4 \mu \Phi _{0}/\sqrt{Hz}≈4μΦ0​/Hz​in both demodulation domains (i.e. amplitudeγ\gammaγand resonator phaseθ\thetaθ) and dominated by the cryogenic microwave chain noise temperature ofTncryo=4.6T_{n}^{cryo}=4.6Tncryo​=4.6K. These results are comparable to those obtained with traditional SDR architectures and demonstrate that the Direct-RF SDR prototype is suitable for the readout ofµµµMUX devices.
This thesis describes a multiplexer of Superconducting Quantum Interference Devices (SQUIDs) with low-noise, ultra-low power dissipation, and great scalability. The multiplexer circuit measures the magnetic flux in a large number of unshunted rf SQUIDs by coupling each SQUID to a superconducting microwave resonator tuned to a unique resonance frequency and driving the resonators from a common feedline. A superposition of microwave tones measures each SQUID simultaneously using only two coaxial cables between the cryogenic device and room temperature. This multiplexer will enable the instrumentation of arrays with hundreds of thousands of low-temperature detectors for new applications in cosmology, materials analysis, and nuclear non-proliferation.\nThe driving application of the Microwave SQUID Multiplexer is the readout of large arrays of superconducting transition-edge sensors, by some figures of merit the most sensitive detectors of electromagnetic signals over a span of more than nine orders of magnitude in energy, from 40 GHz microwaves to 200 keV gamma rays. Modern transition-edge sensors have noise-equivalent power as low as 10-20 W / [square root] Hz and energy resolution as good as 2 eV at 6 keV. These per-pixel sensitivities approach theoretical limits set by the underlying signals, motivating a rapid increase in pixel count to access new science. Compelling applications, like the non-destructive assay of nuclear material for treaty verification or the search for primordial gravity waves from inflation use arrays of these detectors to increase collection area or tile a focal plane.\nWe developed three generations of SQUID multiplexers, optimizing the first for flux noise (0.17 mu Phi0 / [square root] Hz) the second for input current noise (19 pA / [square root] Hz) and the last for practical multiplexing of large arrays of cosmic microwave background polarimeters based on transition-edge sensors. Using the last design we demonstrated multiplexed readout of prototype polarimeters with the performance required for the future development of a large-scale astronomical instrument.
The next generation of cryogenic CMB and submillimeter cameras under development require densely instrumented sensor arrays to meet their science goals. The readout of large numbers (~10,000-100,000 per camera) of sub-Kelvin sensors, for instance as proposed for the CMB-S4 experiment, will require substantial improvements in cold and warm readout techniques. To reduce the readout cost per sensor and integration complexity, efforts are presently focused on achieving higher multiplexing density while maintaining readout noise subdominant to intrinsic detector noise and presenting manageable thermal loads. Highly-multiplexed cold readout technologies in active development include Microwave Kinetic Inductance Sensors (MKIDs) and microwave rf-SQUIDs. Both exploit the high quality factors of superconducting microwave resonators to densely channelize sub-Kelvin sensors into the bandwidth of a microwave transmission line. In the case of microwave SQUID multiplexing, arrays of transition-edge sensors (TES) are multiplexed by coupling each TES to its own superconducting microwave resonator through an rf-SQUID. We present advancements in the development of a new warm readout system for microwave SQUID multiplexing, the SLAC Superconducting Microresonator RF electronics, or SMuRF, by adapting SLAC National Accelerator Laboratory's Advanced Telecommunications Computing Architecture (ATCA) FPGA Common Platform. SMuRF aims to read out 4000 microwave SQUID channels between 4 and 8 GHz per RF line. Each compact SMuRF system is built onto a single ATCA carrier blade. Daughter boards on the blade implement RF frequency-division multiplexing using FPGAs, fast DACs and ADCs, and an analog up- and down-conversion chain. The system reads out changes in flux in each resonator-coupled rf-SQUID by monitoring the change in the transmitted amplitude and frequency of RF tones produced at each resonator's fundamental frequency. The SMuRF system is unique in its ability to track each tone, minimizing the total RF power required to readout each resonator, thereby significantly reducing the linearity requirements on the cold and warm readout. Here, we present measurements of the readout noise and linearity of the first full SMuRF system, including a demonstration of closed-loop tone tracking on a 528 channel cryogenic microwave SQUID multiplexer. SMuRF is being explored as a potential readout solution for a number of future CMB projects including Simons Observatory, BICEP Array, CCAT-prime, Ali-CPT, and CMB-S4. In addition, parallel development of the platform is underway to adapt SMuRF to read out both MKID and fast X-ray TES calorimeter arrays.
Abstract The readout system with a high multiplexing ratio has become a bottleneck limiting the application of large-scale Transition Edge Sensor (TES) detector arrays. In recent years, the microwave superconducting quantum interference device (SQUID) multiplexer has emerged as a key technology for effectively reading large-scale cryogenic detector arrays. Currently, the microwave SQUID multiplexer is being adopted by an increasing number of experiments due to its capability of achieving a multiplexing ratio of 2000:1 within the readout bandwidth. In this study, we developed a 32-channel microwave SQUID multiplexer prototype. And we measured 8 channels of the prototype. The measured equivalent noise current of the prototype reached 154 pA/√ Hz.
Abstract The Microwave SQUID Multiplexer (μ\muμMUX) is a critical readout technology for large-scale Superconducting Transition-Edge Sensor (TES) array. It is considered as a promising readout system for both bolometer and calorimeter applications of large-scale TES array. It has been employed in several Cosmic Microwave Background (CMB) experiments, in which the TESes are used as bolometers. However, in the field of calorimeter applications of TESes, such as X-ray detection, the μ\muμMUX system has not been widely used. The main reason is the slew rate of calorimeter pulse signal is much faster than that of bolometer, which will increase the difficulties of data processing in μ\muμMUX electronics. In a typical microwave SQUID Multiplexed readout scheme, the sampling rate is determined by the frequency of the flux-ramp modulation. For CMB experiment, the frequency of the flux-ramp is usually set to about O\mathcal{O}O(10~kHz). However, for X-ray detection, it is necessary to improve the frequency of flux-ramp to about 1~MHz. It is a challenge to implement the high speed data processing with current μ\muμMUX electronics.%The μ\muμMux readout algorithm is a crucial component.Currently, the implementation of the μ\muμMux readout algorithm is complex, and the sampling rate for each TES is low, only reaching kHz.We propose a recursive algorithm to reconstruct the fast TES signals with high sampling rate in case of no hardware changes. This algorithm is simple enough to implement in real time data collection. %In this paper, we describe and demonstrate this technique with a reasonable simulation model of μ\muμMUX. The simulation results shows that the recovered sampling rate can break through the frequency limitation of the flux ramp, reaching up to analog-to-digital converter (ADC) sampling rate. The non-linearity is lower to 0.25\% in full dynamic range.
We report very-low-noise, fast-response, middle-scale multiplexing in a microwave superconducting quantum interference device multiplexer (MW-Mux) as a transition-edge sensor (TES) readout. Our MW-Mux is able to read 40 channels with 500 kHz sampling and has a low readout noise of 0.9 μΦ0/Hz (where Φ0 is the magnetic flux quantum), equivalent to 9 pA/Hz. By contrast, a multiplexer of less than 10 pixels with 500 kHz sampling and ∼2 μΦ0/Hz readout noise has so far been reported in the literature. Owing to the 500 kHz sampling, our MW-Mux exhibits a fast response to detect a TES pulse with a rise time around 12 μs. We demonstrated simultaneous readout of 38 pixels from an array of x-ray TES microcalorimeters. The measured full-width values at half-maximum spectral resolution ranged from 2.79 to 4.56 eV, with a median value of 3.30 eV at 5.9 keV, including a ∼10% contribution of readout noise, i.e., 0.9–1.7 eV.