Spin states of phosphorus donors in silicon are strong qubit candidates for quantum computing. In the talk, I will introduce how the operation of such a quantum computer is envisioned [Kane, Nature 393, 133, (1998)], and a recently proposed version where qubit control could be performed via electric (as opposed to magnetic) fields [Tosi et al., arXiv:1509.08538v1].  Finally, I will present our theory contribution [Boross et al., http://arxiv.org/abs/1602.03691] describing phonon-emission-mediated spin relaxation processes; we predict that they can be orders-of-magnitude faster in the proposed nanostructured devices than in bulk silicon.