Identification of a telecom wavelength single photon emitter in silicon

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.