  We report the narrow inhomogeneous distribution of the zero-phonon line from
tin-vacancy (SnV) centers in diamond and the overlap of spectra from multiple
separated SnV centers. Photoluminescence excitation spectroscopy measurements
at a cryogenic temperature showed that SnV centers exhibit stable fluorescence
and linewidths close to the Fourier transform-limited linewidth. The
inhomogeneous distribution was as low as ~4 GHz, which enabled the observation
of Sn isotope-dependent resonant frequencies. Owing to the narrow inhomogeneous
distribution, we observed multiple SnV centers showing identical photons with
almost the same wavelength and linewidth. Identical SnV centers were also
observed even in different diamond samples, confirming the reliable fabrication
of the high-quality SnV centers.
