  Controlling the quantum properties of individual fluorescent defects in
silicon is a key challenge towards advanced quantum photonic devices prone to
scalability. Research efforts have so far focused on extrinsic defects based on
impurities incorporated inside the silicon lattice. Here we demonstrate the
detection of single intrinsic defects in silicon, which are linked to a
tri-interstitial complex called W-center, with a zero-phonon line at
1.218${\mu}$m. Investigating their single-photon emission properties reveals
new information about this common radiation damage center, such as its dipolar
orientation and its photophysics. We also identify its microscopic structure
and show that although this defect does not feature electronic states in the
bandgap, Coulomb interactions lead to excitonic radiative recombination below
the silicon bandgap. These results could set the stage for numerous quantum
perspectives based on intrinsic luminescent defects in silicon, such as quantum
integrated photonics, quantum communications and quantum sensing.
