  Scalable quantum processors require high-fidelity universal quantum logic
operations in a manufacturable physical platform. Donors in silicon provide
atomic size, excellent quantum coherence and compatibility with standard
semiconductor processing, but no entanglement between donor-bound electron
spins has been demonstrated to date. Here we present the experimental
demonstration and tomography of universal 1- and 2-qubit gates in a system of
two weakly exchange-coupled electrons, bound to single phosphorus donors
introduced in silicon by ion implantation. We surprisingly observe that the
exchange interaction has no effect on the qubit coherence. We quantify the
fidelity of the quantum operations using gate set tomography (GST), and we use
the universal gate set to create entangled Bell states of the electrons spins,
with fidelity ~ 93%, and concurrence 0.91 +/- 0.08. These results form the
necessary basis for scaling up donor-based quantum computers.
