Probing Qubit Memory Errors at the Part-per-Million Level
M. A. Sepiol, A. C. Hughes, J. E. Tarlton, D. P. Nadlinger, T. G. Ballance, Christopher J. Ballance, T. P. Harty, A. M. Steane, Joseph F. Goodwin, D. M. Lucas · Physical Review Letters · 2019
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 ${\ensuremath{\epsilon}}_{m}$ for a $^{43}{\mathrm{Ca}}^{+}$ trapped-ion qubit in the small-error regime and find ${\ensuremath{\epsilon}}_{m}<{10}^{\ensuremath{-}4}$ for storage times $t\ensuremath{\lesssim}50\text{ }\text{ }\mathrm{ms}$. This exceeds gate or measurement times by three orders of magnitude. Using randomized benchmarking, at $t=1\text{ }\text{ }\mathrm{ms}$ we measure ${\ensuremath{\epsilon}}_{m}=1.2(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, around ten times smaller than that extrapolated from the ${T}_{2}^{*}$ time, and limited by instability of the atomic clock reference used to benchmark the qubit.