  Continuing advancements in quantum information processing have caused a
paradigm shift from research mainly focused on testing the reality of quantum
mechanics to engineering qubit devices with numbers required for practical
quantum computation. One of the major challenges in scaling toward large-scale
solid-state systems is the limited input/output (I/O) connectors present in
cryostats operating at sub-kelvin temperatures required to execute quantum
logic with high-fidelity. This interconnect bottleneck is equally present in
the device fabrication-measurement cycle, which requires high-throughput and
cryogenic characterization to develop quantum processors. Here we multiplex
quantum transport of two-dimensional electron gases at sub-kelvin temperatures.
We use commercial off-the-shelf CMOS multiplexers to achieve an order of
magnitude increase in the number of wires. Exploiting this technology we
advance 300 mm epitaxial wafers manufactured in an industrial CMOS fab to a
record electron mobility of (3.9$\pm$0.6)$\times$10$^5$ cm$^2$\slash Vs and
percolation density of (6.9$\pm$0.4)$\times$10$^{10}$ cm$^{-2}$, representing a
key step toward large silicon qubit arrays. We envision that the demonstration
will inspire the development of cryogenic electronics for quantum information
and because of the simplicity of assembly, low-cost, yet versatility, we
foresee widespread use of similar cryo-CMOS circuits for high-throughput
quantum measurements and control of quantum engineered systems.
