Rare-earth ion doped crystals possess stable solid state physicochemical properties and long optical and spin coherence times, thus displaying important development prospect in quantum information science and technology area. Investigations based on macroscopic bulk rare-earth single crystals have led to many promising results, especially in optical quantum memory studies. With the rapid development of quantum information science, a variety of new functions or multifunctional integrations are expected in rare earth crystal systems, such as on chip quantum storage, microwave to optical frequency conversion, scalable quantum single photon sources and quantum logic gates and so on. As a result,beyond the macroscopic bulk rare-earth single crystals, micro/nano-scale rare-earth crystals attract much attention in recent years and are regarded as promising candidates in highly integrated hybrid quantum systems and miniaturized quantum devices. Moreover, wet chemical method synthesized micro/nano-scale rare-earth crystals have lower growth difficulty and more manipulation flexibility in volume, shape and composition. Therefore, exploring high-performance micro/nano-scale rare-earth crystals and precisely manipulating their quantum states have become one of the important directions in nowadays quantum information science and technology research. In this review, we first introduce briefly the basic concepts and high resolution spectroscopic techniques that commonly used in rare earth ion doped crystals for quantum information science and technologies, such as hole burning and photon echo techniques. We then summarize comprehensively recent research status and development trends of rare earth ion doped polycrystalline nanoparticles, thin films, single crystal based micro systems and some other micro/nano-scale rare earth platforms on terms of material fabrication, quantum coherence property, dephasing mechanisms and also quantum device explorations. Their latest research advances in quantum information applications such as quantum storage, quantum frequency conversion, quantum single photon sources and quantum logic gates are given. Finally, we discuss the possible optimization directions and strategies to improve the component design, material synthesis and quantum performance of micro/nano-scale rare earth crystals and their related quantum devices. This review highlights that micro/nano-scale rare earth crystals may offer many new possibilities for the design of quantum light-matter interfaces, thus are promising quantum systems to develop scalable and integrated quantum devices in the future.
