Characterization of a microwave SQUID multiplexer

Abstract

We consider a novel device for multiplexing superconducting quantum interference devices (SQUIDs) with potential utility in a wide range of applications, including highsensitivity experiments in observational cosmology. We briefly review the basic theory behind the rf SQUID and the current state of multiplexing in cosmology experiments. The microwave multiplexer consists of many different frequency channels, each of which corresponds to a microwave resonator coupled to a SQUID whose flux-dependent inductance detunes the resonant frequency of the channel. We discuss the quantitative details of this SQUID–resonator coupling, and briefly discuss advantages and challenges inherent in this design. We have tested a prototype version of this multiplexer with 35 resonators, most of which are each inductively coupled to a rf SQUID. Even at 4 K, far above the ideal operating temperature of the prototype, we are still able to see resonances and input-dependent detuning in accordance with design, and qualitatively demonstrate multiplexing.