  Rare-earth-doped crystals are excellent hardware for quantum storage of
optical information. Additional functionality of these materials is added by
their waveguiding properties allowing for on-chip photonic networks. However,
detection and coherent properties of rare-earth single-spin qubits have not
been demonstrated so far. Here, we present experimental results on
high-fidelity optical initialization, effcient coherent manipulation, and
optical readout of a single electron spin of Ce$^{3+}$ ion in a YAG crystal.
Under dynamic decoupling, spin coherence lifetime reaches $T_2$=2 ms and is
almost limited by the measured spin-lattice relaxation time $T_1$=3.8 ms.
Strong hyperfine coupling to aluminium nuclear spins suggests that cerium
electron spins can be exploited as an interface between photons and long-lived
nuclear spin memory. Combined with high brightness of Ce$^{3+}$ emission and a
possibility of creating photonic circuits out of the host material, this makes
cerium spins an interesting option for integrated quantum photonics.
