Harnessing the quantum nature of donor atoms in silicon maypave the way for a quantum revolution in the modern digital information era. The idea to combine the exceptional spin coherence properties of donor electron spins in silicon with the prospectof exploiting technology prevalent in the semiconductor industryis very appealing. This thesis provides a quantitative limit forthe spin coherence times of phosphorus donor-bound electrons insilicon, which is a fundamental parameter for spin-based quantum computation. To this end, the spin-lattice relaxation time in28Si:P is measured with the highest degree of precision to date forunprecedentedly low temperatures. The measurements yield extremely long spin-lattice relaxation times exceeding twenty hours,which is orders of magnitude larger than originally determined.These long spin-relaxation times confirm the latent potential fordevices based on spin manipulation donor electrons in silicon. Forvery low temperatures and high magnetic fields, the impact ofthe bosonic phonon distribution on the spin-relaxation time is observed for the very first time and with high accuracy which waspredicted by theory more than 60 years ago.Furthermore, a new method of measuring the bandgap using donorelectrons based on optical spectroscopy of the D0X transition ispresented. This new method can be used to locally detect the lattice temperature via the Si bandgap with exceptional accuracyand excellent temporal resolution. With the help of this method,measurements of the bandgap temperature dependence are performed with 7e-10 relative precision. Although the precisemeasurements verify the theoretical T^4 limit of the bandgap energy shift with high certainty, a discrepancy of the absolute shiftquestions the existing theory of electron-phonon coupling in semi-conductors in the low temperature limit. Additional time-resolvedexperiments facilitate the use of this new method as a precise localthermometer to be used in 28Si:P based devices