  Arbitrarily long quantum computations require quantum memories that can be
repeatedly measured without being corrupted. Here, we preserve the state of a
quantum memory, notably with the additional use of flagged error events. All
error events were extracted using fast, mid-circuit measurements and resets of
the physical qubits. Among the error decoders we considered, we introduce a
perfect matching decoder that was calibrated from measurements containing up to
size-4 correlated events. To compare the decoders, we used a partial
post-selection scheme shown to retain ten times more data than full
post-selection. We observed logical errors per round of
$2.2\pm0.1\times10^{-2}$ (decoded without post-selection) and
$5.1\pm0.7\times10^{-4}$ (full post-selection), which was less than the
physical measurement error of $7\times10^{-3}$ and therefore surpasses a
pseudo-threshold for repeated logical measurements.

Quantum Physics