Highly-multiplexed microwave SQUID readout using the SLAC Microresonator
  Radio Frequency (SMuRF) Electronics for Future CMB and Sub-millimeter Surveys
  The next generation of cryogenic CMB and submillimeter cameras under
development require densely instrumented sensor arrays to meet their science
goals. The readout of large numbers ($\sim$10,000--100,000 per camera) of
sub-Kelvin sensors, for instance as proposed for the CMB-S4 experiment, will
require substantial improvements in cold and warm readout techniques. To reduce
the readout cost per sensor and integration complexity, efforts are presently
focused on achieving higher multiplexing density while maintaining readout
noise subdominant to intrinsic detector noise. Highly-multiplexed cold readout
technologies in active development include Microwave Kinetic Inductance Sensors
(MKIDs) and microwave rf-SQUIDs. Both exploit the high quality factors of
superconducting microwave resonators to densely channelize sub-Kelvin sensors
into the bandwidth of a microwave transmission line. We present advancements in
the development of a new warm readout system for microwave SQUID multiplexing,
the SLAC Superconducting Microresonator RF electronics, or SMuRF. The SMuRF
system is unique in its ability to track each tone, minimizing the total RF
power required to read out each resonator, thereby significantly reducing the
linearity requirements on the cold and warm readout. Here, we present
measurements of the readout noise and linearity of the first full SMuRF system,
including a demonstration of closed-loop tone tracking on a 528 channel
cryogenic microwave SQUID multiplexer. SMuRF is being explored as a potential
readout solution for several future CMB projects including Simons Observatory,
BICEP Array, CCAT-prime, Ali-CPT, and CMB-S4. Parallel development of the
platform is underway to adapt SMuRF to read out both MKID and fast X-ray TES
calorimeter arrays.
