Quantum computer error structure probed by quantum error correction
  syndrome measurements
With quantum devices rapidly approaching qualities and scales needed for
fault tolerance, the validity of simplified error models underpinning the study
of quantum error correction needs to be experimentally evaluated. In this work,
we have assessed the performance of IBM superconducting quantum computer
devices implementing heavy-hexagon code syndrome measurements with increasing
circuit sizes up to 23 qubits, against the error assumptions underpinning code
threshold calculations. Circuit operator change rate statistics in the presence
of depolarizing and biased noise were modelled using analytic functions of
error model parameters. Data from 16 repeated syndrome measurement cycles was
found to be inconsistent with a uniform depolarizing noise model, favouring
instead biased and inhomogeneous noise models. Spatial-temporal correlations
investigated via $Z$ stabilizer measurements revealed significant temporal
correlation in detection events. These results highlight the non-trivial
structure which may be present in the noise of quantum error correction
circuits, revealed by operator measurement statistics, and support the
development of noise-tailored codes and decoders to adapt.
Spiro Gicev Lloyd C. L. Hollenberg Muhammad Usman