KleinProbe: Circuit-Conditioned Hardware Observability for Superconducting Quantum Processors

KleinProbe: Circuit-Conditioned Hardware Observability for Superconducting Quantum Processors KleinProbe is a spatial execution environment sensor for superconducting quantum processors. By executing a structured 18-qubit probe circuit on candidate chip regions, the tool returns syndrome entropy (H) and related statistics that characterize the local execution environment at the time of circuit execution. Key Features and Findings: Stable Spatial Ranking: The spatial ranking of H is stable across calibration cycles, even when absolute values drift. In validation tests, the same chip region consistently ranked first (7/7 runs on ibm_fez; 4/4 runs on ibm_marrakesh), with intra-session stability showing a Spearman ρ = 1.000. Correlation with Hardware Quality: H serves as a circuit-conditioned aggregate of hardware health, showing strong positive correlation with CX gate error (r = +0.746). Furthermore, we observe a very strong correlation between CX_max errors and application circuit quality (r = -0.935). Drift and Era Detection: The BaselineTracker automatically detects fundamental calibration era transitions (e.g., Z = -32.3σ for the ibm_fez Era 1→2 transition) while correctly identifying and rejecting daily within-era fluctuations. Empirical Validation: The framework has been validated on three IBM Heron r2 processors across 54 independent calibration sessions.