  We identify the exact microscopic structure of the G photoluminescence center
in silicon by first principles calculations with including a self-consistent
many-body perturbation method, which is a telecommunication wavelength single
photon source. The defect constitutes of $\text{C}_\text{s}\text{C}_\text{i}$
carbon impurities in its
$\text{C}_\text{s}-\text{Si}_\text{i}-\text{C}_\text{s}$ configuration in the
neutral charge state, where $s$ and $i$ stand for the respective substitutional
and interstitial positions in the Si lattice. We reveal that the observed fine
structure of its optical signals originates from the athermal rotational
reorientation of the defect. We attribute the monoclinic symmetry reported in
optically detected magnetic resonance measurements to the reduced tunneling
rate at very low temperatures. We discuss the thermally activated motional
averaging of the defect properties and the nature of the qubit state.
