High fidelity readout of a single electron spin

We use the two spin states of the valence electron of a single trapped $^{88}$Sr$^{+}$ ion as a physical qubit implementation. For qubit readout one of the qubit states is shelved to a metastable $D$ level using a narrow linewidth 674nm diode laser followed by state-selective fluorescence detection. Careful analysis of the resulting photon detection statistics allows for a minimal detection error of 2 $\cdot $ 10$^{-3}$, compatible with recent estimates of the fault-tolerance required error threshold.