  Color centers in diamond have attracted much interest as candidates for
optically active, solid-state quantum bits. Of particular interest are
inversion-symmetric color centers based on group-IV impurities in diamond
because they emit strongly into their zero-phonon lines and are insensitive to
electric field noise to first order. Early studies of the negatively-charged
tin-vacancy (SnV$^{-}$) center in diamond have found the SnV$^{-}$ to be a
promising candidate: it has high quantum efficiency, emits strongly into its
zero-phonon lines, and is expected to have a long $T_2$ spin coherence time at
4~K. To develop the SnV$^{-}$ into a spin qubit requires further
characterization, especially of the spin and optical properties of individual
SnV$^{-}$ in nanofabricated structures. In this work we isolate single
SnV$^{-}$ centers in diamond nanopillars and characterize their emission
properties and their spin response to a magnetic field. We observe narrow
emission linewidths that are spectrometer-limited, as well as a strong
polarization dependence of each transition. We also find the Zeeman splitting
under a magnetic field to be in good agreement with theoretical prediction. Our
results pave the way toward future employment of single SnV$^{-}$s for
optically accessible quantum memories.
