  Er-doped Si is a promising candidate for quantum information applications due
to its telecom wavelength optical transition and its compatibility with Si
nanofabrication technologies. Recent spectroscopic studies based on
photoluminescence excitation have shown multiple well-defined lattice sites
that Er occupies in Si. Here we report the first measurement of the Zeeman and
hyperfine tensors of a single 167Er3+ ion in Si. All the obtained tensors are
highly anisotropic with the largest value principal axes aligning in nearly the
same direction, and the trace of the lowest crystal field level g-tensor is
17.78$\pm$0.40. The results indicate that this specific Er site is likely to be
a distorted cubic site that exhibits monoclinic (C1) symmetry. Finally, zero
first-order-Zeeman (ZEFOZ) fields are identified for this site and could be
used to reduce decoherence of hyperfine spin states in future experiments.
