  Correlated photon pairs are a fundamental building block of quantum photonic
systems. While pair sources have previously been integrated on silicon chips
built using customized photonics manufacturing processes, these often take
advantage of only a small fraction of the established techniques for
microelectronics fabrication and have yet to be integrated in a process which
also supports electronics. Here we report the first demonstration of
quantum-correlated photon pair generation in a device fabricated in an
unmodified advanced (sub-100nm) complementary metal-oxide-semiconductor (CMOS)
process, alongside millions of working transistors. The microring resonator
photon pair source is formed in the transistor layer structure, with the
resonator core formed by the silicon layer typically used for the transistor
body. With ultra-low continuous-wave on-chip pump powers ranging from 5 $\mu$W
to 400 $\mu$W, we demonstrate pair generation rates between 165 Hz and 332 kHz
using >80% efficient WSi superconducting nanowire single photon detectors.
Coincidences-to-accidentals ratios consistently exceeding 40 were measured with
a maximum of 55. In the process of characterizing this source we also
accurately predict pair generation rates from the results of classical
four-wave mixing measurements. This proof-of-principle device demonstrates the
potential of commercial CMOS microelectronics as an advanced quantum photonics
platform with capability of large volume, pristine process control, and where
state-of-the-art high-speed digital circuits could interact with quantum
photonic circuits.
