On-demand multimode optical storage in a laser-written on-chip waveguide

Quantum memory is a fundamental building block for large-scale quantum networks. On-demand optical storage with a large bandwidth, a high multimode capacity, and an integrated structure simultaneously is crucial for practical application. However, this has not been demonstrated yet. Here, we fabricate an on-chip waveguide in a Eu-151(3+) : Y2SiO5 crystal with insertion losses of 0.2 dB, and propose a pumping scheme to enable spin-wave atomic frequency comb (AFC) storage with a bandwidth of 11 MHz inside the waveguide. Based on this, we demonstrate the storage of 200 temporal modes using the AFC scheme and conditional on-demand storage of 100 temporal modes using the spin-wave AFC scheme. The interference visibility between the readout light field and the reference light field is 99.0% +/- 0.6% and 97% +/- 3% for AFC and spin-wave AFC storage, respectively, indicating the coherent nature of this low-loss, multimode, and integrated storage device.