  Spin-to-photon interfaces from defects in silicon hold great promise towards
realizing quantum repeaters with the combination of advanced semiconductor and
photonics technologies. Recently, controlled creation and erasure of simple
carbon interstitial defects have been successfully realised in silicon. This
defect has a stable structure near room temperature and emits in the
wave-length where the signal loss is minimal in optical fibres used in
communication technologies. Our in-depth theoretical characterization confirms
the assignment of the observed emission to the neutral charge state of this
defect. We find that the emission is due to the recombination of a bound
exciton. We also discovered a metastable triplet state that could be applied as
a quantum memory. Based on the analysis of the electronic structure of the
defect and its similarities to a known optically detected magnetic resonance
centre in silicon, we propose that a carbon interstitial can act as a quantum
bit and may realize a spin-to-photon interface in CMOS-compatible platforms.
