  The ability to distribute quantum entanglement over long distances is a vital
ingredient for quantum technologies. Single atoms and atom-like defects in
solids are ideal quantum light sources and quantum memories to store
entanglement. However, a major obstacle to developing long-range quantum
networks is the mismatch between typical atomic transition energies in the
ultraviolet and visible spectrum, and the low-loss propagation band of optical
fibers in the infrared, around 1.5 $\mu$m. A notable exception is the Er$^{3+}$
ion, whose 1.5 $\mu$m transition is exploited in fiber amplifiers that drive
modern communications networks. However, an optical interface to single
Er$^{3+}$ ions has not yet been achieved because of the low photon emission
rate, less than 100 Hz, that results from the electric dipole-forbidden nature
of this transition. Here, we demonstrate that the emission rate of single
Er$^{3+}$ ions in a solid-state host can be enhanced by a factor of more than
300 using a small mode-volume, low-loss silicon nanophotonic cavity. This
enhancement enables the fluorescence from single Er$^{3+}$ ions to be clearly
observed for the first time. Tuning the excitation laser over a small frequency
range allows dozens of distinct ions to be addressed, and the splitting of the
lines in a magnetic field confirms that the optical transitions are coupled to
the Er$^{3+}$ ions' spins. These results are a significant step towards
long-distance quantum networks and deterministic quantum logic for photons,
based on a scalable silicon nanophotonics architecture.
