  Donor spins in silicon are some of the most promising qubits for upcoming
solid-state quantum technologies. The nuclear spins of phosphorus donors in
enriched silicon have among the longest coherence times of any solid-state
system as well as simultaneous qubit initialization, manipulation and readout
fidelities near ~99.9%. Here we characterize the phosphorus in silicon system
in the regime of "zero" magnetic field, where a singlet-triplet spin clock
transition can be accessed, using laser spectroscopy and magnetic resonance
methods. We show the system can be optically hyperpolarized and has ~10 s Hahn
echo coherence times, even at Earth's magnetic field and below.
