Microwave SQUID Multiplexing of Metallic Magnetic Calorimeters: Status of Multiplexer Performance and Room-Temperature Readout Electronics Development

To our present best knowledge, microwave SQUID multiplexing ( $$\mu $$ MUXing) is the most suitable technique for reading out large-scale low-temperature microcalorimeter arrays that consist of hundreds or thousands of individual pixels which require a large readout bandwidth per pixel. For this reason, the present readout strategy for metallic magnetic calorimeter (MMC) arrays combining an intrinsic fast signal rise time, an excellent energy resolution, a large energy dynamic range, a quantum efficiency close to $$100\%$$ as well as a highly linear detector response is based on $$\mu $$ MUXing. Within this paper, we summarize the state of the art in MMC $$\mu $$ MUXing and discuss the most recent results. This particularly includes the discussion of the performance of a 64-pixel detector array with integrated, on-chip microwave SQUID multiplexer, the progress in flux ramp modulation of MMCs as well as the status of the development of a software-defined radio-based room-temperature electronics which is specifically optimized for MMC readout.