  Bell's theorem sets a boundary between the classical and quantum realms, by
providing a strict proof of the existence of entangled quantum states with no
classical counterpart. An experimental violation of Bell's inequality demands
simultaneously high fidelities in the preparation, manipulation and measurement
of multipartite quantum entangled states. For this reason the Bell signal has
been tagged as a single-number benchmark for the performance of quantum
computing devices. Here we demonstrate deterministic, on-demand generation of
two-qubit entangled states of the electron and the nuclear spin of a single
phosphorus atom embedded in a silicon nanoelectronic device. By sequentially
reading the electron and the nucleus, we show that these entangled states
violate the Bell/CHSH inequality with a Bell signal of 2.50(10). An even higher
value of 2.70(9) is obtained by mapping the parity of the two-qubit state onto
the nuclear spin, which allows for high-fidelity quantum non-demolition
measurement (QND) of the parity. Furthermore, we complement the Bell inequality
entanglement witness with full two-qubit state tomography exploiting QND
measurement, which reveals that our prepared states match the target maximally
entangled Bell states with $>$96\% fidelity. These experiments demonstrate
complete control of the two-qubit Hilbert space of a phosphorus atom, and show
that this system is able to maintain its simultaneously high initialization,
manipulation and measurement fidelities past the single-qubit regime.
