Optical coherence and energy-level properties of a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mi>Tm</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math> -doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Li</mml:mi><mml:mi>Nb</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> waveguide at subkelvin temperatures

We characterize the optical coherence and energy-level properties of 795 nm $^3$H$_6$ to $^3$H$_4$ transition Tm$^{3+}$ in a Ti$^{4+}$:LiNbO$_{3}$ waveguide at temperatures as low 0.65 K. Coherence are measured with varied temperature, magnetic field, excitation power wavelength, measurement time-scale. also investigate nuclear spin-induced hyperfine structure population dynamics varying field laser power. Except for accountable differences due difference Ti$^{4+}$ Tm$^{3+}$-doping concentrations, we find that Tm$^{3+}$:Ti$^{4+}$:LiNbO$_{3}$ produced by indiffusion doping consistent those bulk-doped Tm$^{3+}$:LiNbO$_{3}$ crystal under similar conditions. Our results, which complement previous work narrower parameter space, support using rare-earth-ions integrated quantum signal processing.