Hole burning study of Tm³⁺:YAG hyperfine structure for quantum storage applications

Quantum storage, which aims at transferring photon quantum states into matter, can be obtained by using an ensemble of atoms whose levels form a three level Λ system. In these systems, two optical transitions couple two levels to a third one. This quantum storage scheme could be obtained with rare-earth ions in single crystals, since their optical and ground-state hyperfine transitions can exhibit long coherence lifetimes and their hyperfine structures could be used to build a three level Λ system. Tm ³⁺ ions in Y₃Al₅O₁₂ are especially interesting since the ³H₆-³H₄ transition can be driven by ultra-stable laser diodes. However, the selection rules on the nuclear spin projection can forbid the simultaneous coupling of two levels to a third one. In this paper, the hyperfine structure of Tm ³⁺ is investigated by hole burning spectroscopy under a magnetic field and compared to theoretical calculations based on crystal field calculations. The experimental results are found to be in good agreement with theory and show that some magnetic field orientations are able to relax the selection rules on the nuclear spin projection.