Dc-SQUIDs for the readout of magnetic microcalorimeters

Two-stage current-sensing dc-SQUIDs are presently the devices of choice to read out single-channel magnetic microcalorimeters (MMCs) since they provide quantum-limited noise performance, large system bandwidth and are compatible with operation temperatures well below 100 mK. However, it is very well known that parasitic inductances in the SQUID input circuitry lead to a reduction of the signal size of the detector and that SQUID noise often sets a limit to the energy resolution. In order to minimize these effects, we develop two-stage current-sensing dc-SQUIDs optimized for MMC readout as well as dc-SQUIDs suited for direct temperature sensor readout. In this contribution we discuss our SQUID designs including single second-order gradiometric SQUIDs, N-SQUID series arrays as well as SQUIDs with meander-shaped SQUID inductance that are eligible for direct temperature sensor readout. We show that our SQUIDs exhibit exceptional small low-frequency excess flux noise and are hence very well suited for MMC readout. We further present an integrated setup in which detector and SQUID are located on the same chip to maximize the coupling efficiency of the superconducting flux transformer. Finally, we discuss experimental data of a detector setup with direct temperature sensor readout and prove that this strategy should allow for reaching a sub-eV energy resolution.