Digital Frequency-Domain Multiplexing (DfMux) is a technique that uses MHz superconducting resonators and Superconducting Quantum Interference Device (SQUID) arrays to read out sets of Transition Edge Sensors. DfMux has been used by several Cosmic Microwave Background experiments, including most recently POLARBEAR-2 and SPT-3G with multiplexing factors as high as 68, and is the baseline readout technology for the planned satellite mission LiteBIRD. Here, we present recent work focused on improving DfMux readout noise, reducing parasitic impedance, and improving sensor operation. We have achieved a substantial reduction in stray impedance by integrating the sensors, resonators, and SQUID array onto a single carrier board operated at 250 mK. This also drastically simplifies the packaging of the cryogenic components and leads to better-controlled crosstalk. We demonstrate a low readout noise level of 8.6 pA/Hz−1/2, which was made possible by operating the SQUID array at a reduced temperature and with a low dynamic impedance. This is a factor of two improvement compared to the achieved readout noise level in currently operating Cosmic Microwave Background experiments using DfMux and represents a critical step toward maturation of the technology for the next generation of instruments.
@article{arxiv.2112.02425,
title = {Low Noise Frequency Domain Multiplexing of TES Bolometers using Sub-kelvin SQUIDs},
author = {Tucker Elleflot and Aritoki Suzuki and Kam Arnold and Chris Bebek and Robin H. Cantor and Kevin T. Crowley and John Groh and Tijmen de Haan and Amber Hornsby and John Joseph and Adrian T. Lee and Tiffany Liu and Joshua Montgomery and Megan Russell and Qingyang Yu},
journal= {arXiv preprint arXiv:2112.02425},
year = {2022}
}