  The performance of modular, networked quantum technologies will be strongly
dependent upon the quality of their quantum light-matter interconnects.
Solid-state colour centres, and in particular T centres in silicon, offer
competitive technological and commercial advantages as the basis for quantum
networking technologies and distributed quantum computing. These newly
rediscovered silicon defects offer direct telecommunications-band photonic
emission, long-lived electron and nuclear spin qubits, and proven native
integration into industry-standard, CMOS-compatible, silicon-on-insulator (SOI)
photonic chips at scale. Here we demonstrate further levels of integration by
characterizing T centre spin ensembles in single-mode waveguides in SOI. In
addition to measuring long spin T_1 times, we report on the integrated centres'
optical properties. We find that the narrow homogeneous linewidth of these
waveguide-integrated emitters is already sufficiently low to predict the future
success of remote spin-entangling protocols with only modest cavity Purcell
enhancements. We show that further improvements may still be possible by
measuring nearly lifetime-limited homogeneous linewidths in isotopically pure
bulk crystals. In each case the measured linewidths are more than an order of
magnitude lower than previously reported and further support the view that
high-performance, large-scale distributed quantum technologies based upon T
centres in silicon may be attainable in the near term.
