  Tin-vacancy centers in diamond are promising spin-photon interfaces owing to
their high quantum-efficiency, large Debye-Waller factor, and compatibility
with photonic nanostructuring. Benchmarking their single-photon
indistinguishability is a key challenge for future applications. Here, we
report the generation of single photons with $99.7^{+0.3}_{-2.5}\%$ purity and
$63(9)\%$ indistinguishability from a resonantly excited tin-vacancy center in
a single-mode waveguide. We obtain quantum control of the optical transition
with $1.71(1)$-ns-long $\pi$-pulses of $77.1(8)\%$ fidelity. A modest Purcell
enhancement factor of 12 would enhance the indistinguishability to $95$\%. The
greater than $100$ ms spectral stability shown would then enable strings of up
to $10^6$ identical photons to be generated.
