Towards MMC detector arrays: A microwave SQUID multiplexer

Metallic magnetic calorimeters (MMCs) have shown to be suitable detectors for X-ray photons or massive particles in areas like astronomy, nuclear as well as atomic physics. They provide high energy resolution, high quantum efficiency, large energy bandwidth and linearity. However for many applications large detection areas, high count rates or imaging capabilities are crucial. These requirements can be satisfied by combining single detectors into large detector arrays. Due to constraints on cryogenic wiring, heat load and complexity, it is hardly possible to route thousands of wires from room temperature to the detector array. Thus a technique able to read out a large number of channels with a small number of wires has to be set up. Recently a microwave SQUID multiplexer for the readout of low-temperature detector arrays was proposed. In such a setup every pixel consists of a superconducting λ/4-resonator inductively coupled to an unshunted rf-SQUID that is again inductively coupled to an input coil. A current through this coil is transduced to a change of magnetic flux in the SQUID and thus shifts the circuit's resonance frequency. By capacitively coupling many of those circuits to a common transmission line, injecting a frequency comb and monitoring the phase of each resonator, it is possible to infer the initial detector signal. We discuss an adaption of such a setup for the readout of MMC detector arrays.