LaPO₄:Eu³⁺Monazite Crystals: A Promising Platform for Optical Quantum Technologies

Materials science has an important role to play in the successful establishment of quantum technologies as they are expected to develop suitable material platforms for these highly demanding applications. Among different promising solid-state materials, rare-earth ions embedded in some host crystals show outstandingly narrow optical line widths and long-lived spin states at cryogenic temperatures. In this manuscript, we explore a promising rare-earth (RE)-based material for use in quantum applications: monazite-phase LaPO₄:Eu³⁺ crystals, providing the first comprehensive, high-resolution optical coherence and spin lifetime characterization across the full concentration range, starting from 1% Eu³⁺ and up to the fully stoichiometric compound EuPO₄. The results show optical coherence times of the order 100 μs and nuclear spin lifetimes up to 2 h at 4 K, setting this material among the best reported so far, while experimental conditions for its synthesis with appropriate quality consist of a rather simple solid-state reaction. This opens promising perspectives for its use in quantum applications.