The Systolic Drift Monitor: A Hardware Safety Layer for Non-Markovian Noise in Fault-Tolerant Quantum Architectures

Description This work introduces the Systolic Drift Monitor (SDM), a universal hardware safety layer designed to address a fundamental limitation in real-time quantum error correction (QEC): the inability of finite-window decoders to detect slow, non-Markovian drift such as 1/f noise. We formalize this limitation as the Lag Horizon, an information-theoretic blind spot arising from finite memory and boundary-only observation. Using the Fisher-Blindness No-Go Theorem, we show that when error correlations extend beyond the decoder window, the Fisher information available to any windowed observer vanishes, rendering structural phase transitions statistically undetectable regardless of processing speed. To resolve this blindness, we propose the SDM as a minimal, architecture-independent hardware primitive that operates alongside—rather than within—the primary decoder. Implemented on 2025-era FPGA platforms (AMD Versal ACAP and Intel Agilex), the SDM performs deterministic, infinite-horizon integration of syndrome flux, restoring visibility to topological drift while maintaining constant latency. We analyze two experimentally relevant regimes: Superconducting dual-rail qubits, where the SDM functions as an asynchronous drift watchdog for long-timescale flux noise (>100 μs) despite sub-microsecond QEC cycles. Neutral atom processors, where large cycle times (>1 ms) allow the SDM to operate as a hard safety interlock, converting atom loss and leakage precursors into flagged erasures before logical failure. Simulation-based validation across surface codes and color codes demonstrates pre-symptomatic detection of non-Markovian faults and negligible hardware overhead (