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 μm. A notable

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... ception is the Er^3+ ion, whose 1.5 μm 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.