Microwave SQUID multiplexer

We describe a superconducting quantum interference device (SQUID) multiplexer operated at microwave frequencies. The outputs of multiple SQUIDs are simultaneously modulated at different frequencies and summed into the input of one high electron mobility transistor (HEMT). The large bandwidth and dynamic range provided by HEMT amplifiers should make it possible to frequency-division multiplex a large number of SQUIDs in one output coaxial cable. We measure low SQUID noise (similar to0.5 muPhi(0)/rootHz at 4 K) and demonstrate the multiplexed readout of two direct current (dc) SQUIDs at different resonant frequencies. In this work, dc SQUIDs are used, but this approach is equally applicable to radio-frequency SQUIDs.