LAUSR

Single photon emitters in the form of rare-earth (RE) impurities in garnet minerals are being actively explored as solid-state spin-qubits, largely due to their electronically screened 4f states that result in hourslong coherence lifetimes and, as in the case of Er, homogeneous linewidths as narrow as 50 Hz in the telecom band. In addition, these systems show promise for nanoscale sensing and as long-term quantum memories, with storage times exceeding a second. Despite the intrinsic potential of RE ions, most host materials bring in unwanted interactions, particularly in the form of deleterious hyperfine couplings between the ion and surrounding spinactive nuclei and/or impurities. Dynamic decoupling techniques have been adapted to mitigate this problem, though at the expense of an increased complexity in the control protocols. Isotopic depletion of spin-active nuclei can arguably prove effective but this route is not viable for all atomic species (e.g., yttrium), and material growth tends to be costly, particularly if the host contains heavy elements. Even in the case of materials where hyperfine couplings are less critical (such as Er:YSO or Nd:YVO4) the problem of interfacing RE spins with optoelectronic elements remains a challenge as these hosts are not necessarily compatible with present material processing methods. All in all, these obstacles impose serious constraints for quantum