Recently, we showed the first high-resolution spectroscopy and manipulation of single Pr3+ ions in the solid state using rare-earth doped crystals. The special feature of these systems is access to the hyperfine splitting caused by the interaction of the 4f electrons and the nucleus, leading to sets of sublevels with exceptionally long coherence times at cryogenic temperatures, on the order of minutes and hours. This ground state splitting can serve as a lambda level scheme, a key ingredient for many prospective applications in quantum optics, in particular for qubit storage and manipulation. In this presentation, we discuss our latest results and future challenges in reducing spectral diffusion and line broadening down to the expected natural linewidth, prospects for the use of microcavities or plasmonic nano-antennas to enhance the photon yield, and schemes for on-chip integration.
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