Towards Rare-Earth Molecular Crystals as a New Platform in Quantum Networks

Rare-Earth (RE) atoms possess a unique atomic energy level structure, leading to excellent electronic and spin coherence properties [1]. Consequently, RE-ion-doped materials are promising candidates for implementing light-matter interfaces for photonic quantum technologies, such as quantum memories. To exploit these properties in quantum technologies, the REs must be hosted in a solid-state material [1]. RE organic molecular crystals offer advantages over the well-known RE-doped inorganic crystals. In organic molecular crystals, RE ions are integral parts of the molecular structure, unlike in inorganic crystals, in which the RE ions are distributed randomly into the crystal lattice structure. This can potentially lead to narrower inhomogeneous broadening. These crystals provide unmatched flexibility in combining with photonic structures and in material composition, allowing the choice of different ligands and point group symmetries around the RE ion [2], [10], [4]. Our research focuses on crystals formed from a molecular complex hosting erbium ions <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lceil{E}r(C_{5}H_{7}O_{2})_{3}(C_{12}H_{8}N_{2})\rceil$</tex>. As these crystals have not yet been studied for quantum information purposes, we conducted a series of optical spectroscopy measurements to characterize the platform. We performed absorption measurements and obtained the absorption profiles at room and cryogenic temperatures. The results show a narrow inhomogeneous broadening in samples with different relative concentrations, even at 100%, for which all molecules contain an erbium ion. We measured the lifetime of the second lowest crystal field level of the ground state manifold <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\boldsymbol{30}\ cm^{-1}$</tex> above ground state) to be 1.759 seconds. Additionally, we measured the inhomogeneous broadening of the lowest crystal field level versus temperature to investigate the effects of phonon processes. Finally, under an external magnetic field, we observed Zeeman splitting and determined the g-factor to be 3.23 for erbium ions in this molecular complex.