  We characterize the 795 nm $^3$H$_6$ to $^3$H$_4$ transition of Tm$^{3+}$ in
a Ti$^{4+}$:LiNbO$_{3}$ waveguide at temperatures as low as 800 mK. Coherence
and hyperfine population lifetimes -- up to 117 $\mu$s and 2.5 hours,
respectively -- exceed those at 3 K at least ten-fold, and are equivalent to
those observed in a bulk Tm$^{3+}$:LiNbO$_{3}$ crystal under similar
conditions. We also find a transition dipole moment that is equivalent to that
of the bulk. Finally, we prepare a 0.5 GHz-bandwidth atomic frequency comb of
finesse $>$2 on a vanishing background. These results demonstrate the
suitability of rare-earth-doped waveguides created using industry-standard
Ti-indiffusion in LiNbO$_3$ for on-chip quantum applications.
