Digital frequency multiplexing with sub-Kelvin SQUIDs

Digital frequency multiplexing (dfMux) is a readout architecture for transition edge sensor-based detector arrays and is used on telescopes including SPT-3G, POLARBEAR-2, and LiteBIRD. Here, we present recent progress and plans for development of a sub-Kelvin SQUID architecture for digital frequency multiplexed bolometers. This scheme moves the SQUID from the 4K stage to the 250mK stage, adjacent to the bolometers. Operating the SQUID on the detector stage may offer lower noise and greater scalability. Electrical performance will be improved as a result of decreased wiring length and reduced parasitics, allowing for higher multiplexing factors and lower bolometer Rnormal. These performance improvements will enable ultra-large focal planes for future instruments such as CMB-S4.