  We present a hybrid microwave SQUID multiplexer that combines two
frequency-division multiplexing techniques to allow multiplexing a given number
of cryogenic detectors with only a fraction of frequency encoding resonators.
Similar to conventional microwave SQUID multiplexing, our multiplexer relies on
inductively coupling non-hysteretic, unshunted rf-SQUIDs to superconducting
microwave resonators as well as applying flux ramp modulation for output signal
linearization. However, instead of utilizing one resonator per SQUID, we couple
multiple SQUIDs to a common readout resonator and encode the SQUID input
signals in sidebands of the microwave carrier by varying the flux ramp
modulation frequency for each SQUID. We prove the suitability of our approach
using a prototype device and argue by means of fundamental information theory
that our approach is particularly suited for reading out large cryogenic
bolometer arrays.
