  The combination of established nanofabrication with attractive material
properties makes silicon a promising material for quantum technologies, where
implanted dopants serve as qubits with high density and excellent coherence
even at elevated temperatures. In order to connect and control these qubits,
interfacing them with light in nanophotonic waveguides offers unique promise.
Here, we present resonant spectroscopy of implanted erbium dopants in such
waveguides. We overcome the requirement of high doping and above-bandgap
excitation that limited earlier studies. We thus observe erbium incorporation
at well-defined lattice sites with a thousandfold reduced inhomogeneous
broadening of about 1 GHz and a spectral diffusion linewidth down to 45 MHz.
Our study thus introduces a novel materials platform for the implementation of
on-chip quantum memories, microwave-to-optical conversion, and distributed
quantum information processing, with the unique feature of operation in the
main wavelength band of fiber-optic communication.
