  We study the possibility of using single rare-earth ions coupled to a
photonic cavity with high cooperativity for performing non-destructive
measurements of photons, which would be useful for global quantum networks and
photonic quantum computing. We calculate the achievable fidelity as a function
of the parameters of the rare-earth ion and photonic cavity, which include the
ion's optical and spin dephasing rates, the cavity linewidth, the single photon
coupling to the cavity, and the detection efficiency. We suggest a promising
experimental realization using current state of the art technology in
Nd:YVO$_4$.
