  Solid-state emitters with atom-like optical and magnetic transitions are
highly desirable for efficient and scalable quantum state engineering and
information processing. Quantum dots, color centers and impurities embedded in
inorganic hosts have attracted a great deal of attention in this context, but
influences from the matrix continue to pose challenges on the degree of
attainable coherence in each system. We report on a new solid-state platform
based on the optical detection of single praseodymium ions via 4f intrashell
transitions, which are well shielded from their surroundings. By combining
cryogenic high-resolution laser spectroscopy with fluorescence microscopy, we
were able to spectrally select and spatially resolve individual ions. In
addition to elaborating on the essential experimental steps for achieving this
long-sought goal, we demonstrate state preparation and read out of the three
ground-state hyperfine levels, which are known to have lifetimes of the order
of hundred seconds.
