  Optically-interfaced spins in the solid state are a promising platform for
quantum technologies. A crucial component of these systems is high-fidelity,
projective measurement of the spin state. In previous work with laser-cooled
atoms and ions, and solid-state defects, this has been accomplished using
fluorescence on an optical cycling transition; however, cycling transitions are
not ubiquitous. In this work, we demonstrate that modifying the electromagnetic
environment using an optical cavity can induce a cycling transition in a
solid-state atomic defect. By coupling a single Erbium ion defect to a
telecom-wavelength silicon nanophotonic device, we enhance the cyclicity of its
optical transition by a factor of more than 100, which enables single-shot
quantum nondemolition readout of the ion's spin with 94.6% fidelity. We use
this readout to probe coherent dynamics and relaxation of the spin. This
approach will enable quantum technologies based on a much broader range of
atomic defects.
