Null Controls and Replication Limits of Leakage-Sensitive Interrupted-Tag Readout in a Closed-Excursion fSim-GNS Pilot

his technical addendum reports a v0.10 null-control and replication sequence for leakage-sensitive interrupted-tag readout in a closed-excursion fSim-GNS pilot on a superconducting q11–q12 pair. The study was designed to test whether a previously observed interrupted-tag response could be separated from tag-only disturbance, weak off-branch flux sensitivity, and ordinary computational-subspace response. The sequence proceeded through five stages. First, a nominal weak off-branch amplitude control was found not to be a clean null. Second, a clean-null selector using zero-amplitude and weak-amplitude placeholder branches identified quieter control conditions and a near-threshold partner-channel response near 104 ns. Third, a 104 ns state-control add-on produced one prep11-selective partner-channel hit satisfying the pre-specified state-selective criterion. Fourth, an immediate replication failed to reproduce this hit. Fifth, a local timing re-scan over 96–112 ns did not recover a partner-channel or p11 peak. The integrated result supports a conservative interpretation: the interrupted-tag protocol can produce structured leakage/tag-sensitive responses under some conditions, but the strongest state-selective hit was intermittent or drift-sensitive rather than a robust fixed-time qutrit-level signature. The present work does not perform qutrit-resolved readout and does not directly observe |02⟩ or |20⟩. It therefore reports an operational measurement-design boundary: weak control-side tagging with standard binary readout is insufficient for stable direct identification of transient qutrit-level participation on this cloud-accessible device. The recommended next direction is to move from weakly perturbing hidden leakage components to actively mapping leakage participation into measurement-accessible computational-basis differences, for example through partner-targeted tagging, calibrated |2⟩→|1⟩ mapping, or qutrit-resolved readout. Keywords:superconducting qubits; transmon qutrits; leakage tagging; interrupted readout; fSim; fast flux; closed excursion; SU(3); qutrit leakage; null controls; replication; state-control; quantum control; TIST-QC; GNS