Lanthanide‐Based Quantum Optical Materials

Abstract Quantum optical materials are fundamental infrastructure to realize emerging quantum technologies for quantum communication, quantum computation, quantum memory, and sensing. Among the wide range of solid‐state platforms explored, lanthanide‐based systems stand out for their ability to combine atomic‐like coherence with the scalability of condensed matter. The shielded 4f orbitals of lanthanide ions yield long‐lived radiative transitions, narrow homogeneous linewidths, and rich hyperfine structures that support optical and spin coherence extending from milliseconds to hours. These intrinsic properties, combined with the versatility of host environments—from bulk crystals and thin films to fibers and nanocrystals—enable lanthanides to address three pillars of quantum optics: collective emission, single‐ion emission, and ensemble‐based quantum memories. Recent experiments have demonstrated room‐temperature superfluorescence in nanocrystals, single‐ion emission in the telecom band, and quantum memories with record‐setting storage times and efficiencies. Here, a critical review of these advances, emphasizing how control parameters such as host lattice, isotopic purification, dopant concentration, and photonic integration govern performance metrics of lanthanide‐based quantum optical materials, is provided. By analyzing the state‐of‐the‐art of lanthanides for quantum optics and envisioning the potential future directions, the principles to design lanthanide‐based materials as indispensable building blocks for scalable, application‐driven quantum technologies are aimed to highlight.