  Owing to their long excited state lifetimes, rare-earth ions in crystals are
widely used in quantum applications. To allow optical readout of the qubit
state of individual ions, we propose to dope the crystal with an additional
nearby ancilla ion with a shorter radiative lifetime. We show how a Bayesian
analysis exhausts the information about the state of the qubit from the optical
signal of the ancilla ion. We study the effects of incoherent processes and
propose ways to reduce their effect on the readout. Finally, we extend the
architecture to ions residing in two remote cavities, and we show how
continuous monitoring of fluorescence signals from the two ancilla ions leads
to entanglement of the qubit ions.
