Photonic Integrated Quantum Memory in Rare-Earth Doped Solids

An optical quantum memory is a device that can store photonic quantum information and release it after a controlled time. It is an essential component for overcoming channel losses in large-scale quantum networks. Optical quantum memories have been demonstrated with various physical systems including atomic gases, single atoms in optical cavities, and rare-earth-ion doped solids. Now, quantum memories are marching toward miniaturization and integration for large-scale practical applications. Solid state systems stand as a natural choice due to the physical stability and ease of micro or nano fabrication using well-established techniques. In the past decade, considerable efforts have been devoted to developing photonic integrated quantum memories, that is, quantum memories based on micro/nano-photonic structures manufactured in solids. Remarkable performances have been achieved with integrated quantum memories, with the advantages of lower laser/electric power requirements, small volumes, large storage densities, and easy implementations. In this article, the basic concepts of optical quantum memories, the state-of-the-art technologies for fabricating integrated quantum memories in rare-earth ions doped crystals, and recent advances are introduced, and the roadmap for developing practically useful devices for applications in quantum networks is discussed.