An experimental platform for ultralow-temperature pulsed ENDOR spectroscopy is constructed for general bulk materials. Coherent properties of the coupled electron and nuclear spins of rare-earth dopants in a crystal (${}^{143}{\mathrm{Nd}}^{3+}:{\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$) are investigated from 100 mK to 6 K. At the lowest working temperatures, two-pulse-echo coherence times exceeding 2 and 40 ms are achieved for the electron and nuclear spins, while the electronic Zeeman and hyperfine population lifetimes are more than 15 s and 10 min. With the aid of the near-unity electron spin polarization at 100 mK, the complete hyperfine-level structure with 16 energy levels is measured by the ENDOR technique without the assistance of the reconstructed spin Hamiltonian. These results demonstrate the suitability of deeply cooled paramagnetic rare-earth ions for memory components aimed at quantum communication and quantum computation. The experimental platform developed is expected to be a powerful tool for study of paramagnetic materials in various research fields.

• Received 13 January 2019
• Revised 15 January 2020
• Accepted 22 January 2020

DOI:https://doi.org/10.1103/PhysRevApplied.13.024080