Properties of a rare-earth-ion-doped waveguide at sub-Kelvin temperatures for quantum signal processing

Abstract

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.

6 pages, 4 figures