Microwave SQUID multiplexers for low-temperature detectors

We report on the development of a SQUID multiplexer operated at microwave frequencies. This multiplexer can accommodate any detector that can be read out with SQUIDs, including transition-edge sensors, magnetic calorimeters, and superconductor-insulator-superconductor tunnel junction detectors. The total bandwidth, which is set by the HEMT amplifier, can be as high as 5GHz, allowing the readout of 103-104 low-bandwidth detectors into one coaxial cable. Alternatively, in a low-Q circuit each SQUID can have a bandwidth as high as 100-200MHz, allowing multiplexing of high-bandwidth sensors, such as optical TES photon detectors and antenna-coupled hot-electron TES bolometers. Each SQUID must be flux biased at a sensitive point on its response curve. We describe a multiplexed flux biasing scheme that makes it possible to flux bias a large array of SQUIDs with only one additional coaxial cable. We further present a hybrid 'TDMA' multiplexer optimized for the high dynamic range and slew rate of fast X-ray calorimeters