  Rare-earth related electron spins in crystalline hosts are unique material
systems, as they can potentially provide a direct interface between telecom
band photons and long-lived spin quantum bits. Specifically, their optically
accessible electron spins in solids interacting with nuclear spins in their
environment are valuable quantum memory resources. Detection of nearby
individual nuclear spins, so far exclusively shown for few dilute nuclear spin
bath host systems such as the NV center in diamond or the silicon vacancy in
silicon carbide, remained an open challenge for rare-earths in their host
materials, which typically exhibit dense nuclear spin baths. Here, we present
the electron spin spectroscopy of single Ce$^{3+}$ ions in a yttrium
orthosilicate host, featuring a coherence time of $T_{2}=124\,\mu$s. This
coherent interaction time is sufficiently long to isolate proximal $^{89}$Y
nuclear spins from the nuclear spin bath of $^{89}$Y. Furthermore, it allows
for the detection of a single nearby $^{29}$Si nuclear spin, native to the host
material with ~5% abundance. This study opens the door to quantum memory
applications in rare-earth ion related systems based on coupled environmental
nuclear spins, potentially useful for quantum error correction schemes.
