  Large arrays of cryogenic detectors, including transition-edge sensors (TESs)
or magnetic micro-calorimeters (MMCs), are needed for future experiments across
a wide range of applications. Complexities in integration and cryogenic wiring
have driven efforts to develop cryogenic readout technologies with large
multiplexing factors while maintaining minimal readout noise. One such example
is the microwave SQUID multiplexer ($\mu$mux), which couples an incoming TES or
magnetic calorimeter signal to a unique GHz-frequency resonance that is
modulated in frequency. Here, we present a hybrid scheme combining the
microwave SQUID multiplexer with code division multiplexing: the
impedance-modulated code-division multiplexer (Z-CDM), which may enable an
order of magnitude increase in multiplexing factor particularly for
low-bandwidth signal applications.
