  We present the conceptual design of the modular detector and readout system
for the Cosmic Microwave Background Stage 4 (CMB-S4) ground-based survey
experiment. CMB-S4 will map the cosmic microwave background (CMB) and the
millimeter-wave sky to unprecedented sensitivity, using 500,000 superconducting
detectors observing from Chile and Antarctica to map over 60 percent of the
sky. The fundamental building block of the detector and readout system is a
detector module package operated at 100 mK, which is connected to a readout and
amplification chain that carries signals out to room temperature. It uses
arrays of feedhorn-coupled orthomode transducers (OMT) that collect optical
power from the sky onto dc-voltage-biased transition-edge sensor (TES)
bolometers. The resulting current signal in the TESs is then amplified by a
two-stage cryogenic Superconducting Quantum Interference Device (SQUID) system
with a time-division multiplexer to reduce wire count, and matching
room-temperature electronics to condition and transmit signals to the data
acquisition system. Sensitivity and systematics requirements are being
developed for the detector and readout system over a wide range of observing
bands (20 to 300 GHz) and optical powers to accomplish CMB-S4's science goals.
While the design incorporates the successes of previous generations of CMB
instruments, CMB-S4 requires an order of magnitude more detectors than any
prior experiment. This requires fabrication of complex superconducting circuits
on over 10 square meters of silicon, as well as significant amounts of
precision wiring, assembly and cryogenic testing.
