Reliable coherent optical memory based on a laser-written waveguide

Abstract

${}^{151}\mathrm{E}{\mathrm{u}}^{3+}$-doped yttrium silicate (${}^{151}\mathrm{E}{\mathrm{u}}^{3+}:{\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5$ ) crystal is a unique material that possesses hyperfine states with coherence time up to 6 h. Many efforts have been devoted to the development of this material as optical quantum memories based on the bulk crystals, but integrable structures (such as optical waveguides) that can promote ${}^{151}\mathrm{E}{\mathrm{u}}^{3+}:{\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5$-based quantum memories to practical applications, have not been demonstrated so far. Here we report the fabrication of type 2 waveguides in a ${}^{151}\mathrm{E}{\mathrm{u}}^{3+}:{\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5$ crystal using femtosecond-laser micromachining. The resulting waveguides are compatible with single-mode fibers and have the smallest insertion loss of $4.95dB$. On-demand light storage is demonstrated in a waveguide by employing the spin-wave atomic frequency comb (AFC) scheme and the revival of silenced echo (ROSE) scheme. We implement a series of interference experiments based on these two schemes to characterize the storage fidelity. Interference visibility of the readout pulse is $0.99\pm 0.03$ for the spin-wave AFC scheme and $0.97\pm 0.02$ for the ROSE scheme, demonstrating the reliability of the integrated optical memory.