  Silicon is the most developed electronic and photonic technological platform
and hosts some of the highest-performance spin and photonic qubits developed to
date. A hybrid quantum technology harnessing an efficient spin-photon interface
in silicon would unlock considerable potential by enabling ultra-long-lived
photonic memories, distributed quantum networks, microwave to optical photon
converters, and spin-based quantum processors, all linked using integrated
silicon photonics. However, the indirect bandgap of silicon makes
identification of efficient spin-photon interfaces nontrivial. Here we build
upon the recent identification of chalcogen donors as a promising spin-photon
interface in silicon. We determined that the spin-dependent optical degree of
freedom has a transition dipole moment stronger than previously thought (here
1.96(8) Debye), and the T1 spin lifetime in low magnetic fields is longer than
previously thought (> 4.6(1.5) hours). We furthermore determined the optical
excited state lifetime (7.7(4) ns), and therefore the natural radiative
efficiency (0.80(9) %), and by measuring the phonon sideband, determined the
zero-phonon emission fraction (16(1) %). Taken together, these parameters
indicate that an integrated quantum optoelectronic platform based upon
chalcogen donor qubits in silicon is well within reach of current capabilities.
