Full-Scale Microwave SQUID Multiplexer Readout System for Magnetic Microcalorimeters

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.