Highly-multiplexed microwave SQUID readout using the SLAC Microresonator Radio Frequency (SMuRF) electronics for future CMB and sub-millimeter surveys

Abstract

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, which is built on the 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 read out 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.

Microwave SQUID multiplexing for the Lynx x-ray microcalorimeter

“…A number of instruments under development plan to track the resonance frequency of each resonator using a feedback loop instead of using fixed frequency probe tones. 36,38 Since the probe tone spends more time deeper in the resonance less power per tone is transmitted to the amplifiers and mixers in the readout chain. This has the advantage of lowering the linearity requirements for the amplifiers and mixers.…”

BICEP Array: a multi-frequency degree-scale CMB polarimeter

“…The 220/270 GHz receiver, with more than 22,000 detector channels, will require a new readout technology. Thermal kinetic inductance detectors 17 (TKID), and microwave SQUID multiplexing 18 (µMUX) technologies provide the higher multiplexing factors to handle such large formats and are being developed for the high frequency receiver within the collaboration.…”

2017 upgrade and performance of BICEP3: a 95GHz refracting telescope for degree-scale CMB polarization

“…Bicep3 has demonstrated stable performance and serves as a stepping stone for the upgrade of the Keck Array to the BicepArray, an array of four Bicep3-size receivers observing at 30/40, 95, 150, and 220/270 GHz. 5,11 Research and development is ongoing, including the window design, 10 module design, 12 and readout technology, 13,14 which are included in this volume of proceedings.…”