  An experimental platform of ultralow-temperature pulsed ENDOR
(electron-nuclear double resonance) spectroscopy is constructed for the bulk
materials. Coherent property of the coupled electron and nuclear spins of the
rare-earth (RE) dopants in a crystal (143Nd3+:Y2SiO5) is investigated from 100
mK to 6 K. At the lowest working temperatures, two-pulse-echo coherence time
exceeding 2 ms 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 using
ENDOR technique without the assistance of the reconstructed spin Hamiltonian.
These results demonstrate the suitability of the deeply cooled paramagnetic
RE-doped solids for memory components aimed for quantum communication and
quantum computation. The developed experimental platform is expected to be a
powerful tool for paramagnetic materials from various research fields.
