Direct-current superconducting quantum interference devices for the readout of metallic magnetic calorimeters

In this paper we analyze the influence of the coupled energy sensitivity epsilon(c)(f) of a superconducting quantum interference device (SQUID) on the energy resolution of metallic magnetic calorimeters. From this, we derive an upper limit on the readout noise that still allows for the readout of detectors with sub-eV energy resolution. Furthermore, we present two dc-SQUID designs, namely a first-stage SQUID and an N-SQUID series array, that are suited for the readout of high-resolution detectors. We show that fabricated SQUIDs have a noise performance that is competitive to the best state-of-the-art dc-SQUIDs. For the first-stage SQUIDs, we found a correlation between the 1/f noise exponent alpha and the 1/f noise prefactor epsilon(1/f) (1 Hz). Using both kind of SQUIDs we have built a two-stage dc-SQUID configuration. We show that this setup allows for the readout of x-ray detectors with a resolving power E/Delta E-FWHM > 3000.