| Summary: | <p>Robust qubit memory is essential for quantum computing, both for near-term devices operating without error correction, and for the long-term goal of a fault-tolerant processor. We directly measure the memory error ε<sub>m</sub> for a <sup>43</sup>Ca<sup>+</sup> trapped-ion qubit in the small-error regime and find ε<sub>m</sub><10<sup>−4</sup> for storage times <em>t</em> ≲ 50 ms. This exceeds gate or measurement times by three orders of magnitude. Using randomized benchmarking, at <em>t</em> = 1 ms we measure ε<sub>m</sub>=1.2(7)×10<sup>−6</sup>, around ten times smaller than that extrapolated from the T<sup>∗</sup><sub>2</sub> time, and limited by instability of the atomic clock reference used to benchmark the qubit.</p>
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