From Round-Trip State Echo to Error Recovery: Snapshot-Resolved Quantum-Hardware Diagnostics

Abstract

We examine which conclusions from quantum-hardware benchmarks persist when the same programs are run again. On eight fixed routes of IQM Emerald, round-trip state echo (RTSE), a diagnostic of state return after a round trip, gave mean scores near $0.84$ that differed by only $0.00125$ between two executions. Individual route estimates nevertheless changed by up to $0.06$ in both directions. The mean difference between RTSE and measurement-free teleportation along the same routes shrank from $0.046$ to $0.014$ and was no longer statistically resolved on repetition; none of the four prespecified communication targets met its replication rule. Recovery with an existing four-qubit single-deletion code also changed on repetition: after discarding one encoded qubit in the circuit model, the IQM mean fell from $0.738$ to $0.624$ and failed the replication test against the two-thirds operational reference. In a separate IonQ experiment, recovery means of $0.91$ and $0.92$ passed tests against that reference and against a deliberately incorrect decoder in both executions. These results concern the tested workloads and executions. They show why a nearly unchanged aggregate benchmark score needs to be interpreted alongside individual routes, direct task comparisons, and repeated executions.

arXiv comment42 pages, 11 figures, 10 tables. Revised abstract, exposition and interpretation of repeated tests; expanded provenance and reproducibility checks. Original hardware data unchanged. Reproducibility package: doi:10.5281/zenodo.21969397