Reliable coherent optical memory based on a laser-written waveguide

$ ^{{151}}{{\rm Eu}^{{3} + }} $151Eu3+-doped yttrium silicate ($ {^{{151}}{{\rm Eu}}^{{3} + }}:{{\rm Y}_{2}}{{\rm SiO}_{5}} $151Eu3+:Y2SiO5) 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 bulk crystals, but integrable structures (such as optical waveguides) that can promote $ {^{{151}}{{\rm Eu}}^{{3} + }}:{{\rm Y}_{2}}{{\rm SiO}_{5}} $151Eu3+:Y2SiO5-based quantum memories to practical applications have not been demonstrated so far. Here we report the fabrication of type II waveguides in a $ {^{{151}}{{\rm Eu}^{{3} }+ }}:{{\rm Y}_{2}}{{\rm SiO}_{5}} $151Eu3+:Y2SiO5 crystal using femtosecond-laser micromachining. The resulting waveguides are compatible with single-mode fibers and have the smallest insertion loss of 4.95 dB. 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 $0.99±0.03 for the spin-wave AFC scheme and $ 0.97 \pm 0.02 $0.97±0.02 for the ROSE scheme, demonstrating the reliability of the integrated optical memory.