We demonstrate a method of qubit state preparation and measurement (SPAM) based on a weakly open quantum channel that achieves a higher fidelity than previous demonstrations, even with low detection efficiency. The trapped-ion qubit SPAM infidelity ($2.1_{-0.4}^{+0.5}\times {10}^{-4}$) is limited by setup-specific errors from ion loss and imperfect population transfer between qubit eigenstates, and we show that full transfer would yield an inaccuracy less than $8\times {10}^{-5}$. The high precision of this method revealed errors caused by a rare ($\approx {10}^{-4}$ likelihood) magnetic-dipole decay that we measure and correct by driving an additional transition. Since this scheme allows fluorescence collection for effectively unlimited periods, high fidelity is achievable even with limited optical access and low quantum efficiency photon detectors.

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• Quantum control
• Quantum information with trapped ions
• Trapped ions