这篇论文揭示了量子电路合成里Transformer的一个关键问题：自回归漂移让长电路精确匹配率从88%几乎归零，即使数据翻倍也只能部分缓解，值得搞量子计算和AI交叉的人看。
本文研究用44.8M参数的编码器-解码器Transformer进行量子电路合成。在参数化电路（2-6 qubits）上，混合方法（结构来自Transformer，角度来自经典优化）对3-6 qubit电路实现中位数保真度1.000。在Clifford+T电路（3-6 qubits）上，模型虽能学习有效语法和准确T计数，但精确等价率从≤9门的88%跌至超过26门时接近零。推理时生成多候选并通过等价验证选择，将精确匹配率从7%提升至22.5%；训练数据扩展2.5倍则提升至39.5%，但长电路退化仍存在。结论：当需要完全离散正确性时，自回归漂移限制可靠性，推理时搜索和数据扩展有效，而微调和模型多样化无效。
When Close Enough Is Not Enough: Autoregressive Drift in Quantum Circuit Synthesis
Quantum circuit optimization for fault-tolerant computing requires exact functional equivalence while minimizing expensive non-Clifford resources such as T gates. We study this problem using a compact 44.8M-parameter encoder-decoder transformer with structured circuit tokenization, evaluating on parameterized circuits (2-6 qubits) and Clifford+T circuits (3-6 qubits). On parameterized circuits, a hybrid approach -- structure from the transformer, angles from classical optimization -- achieves median fidelity 1.000 on 3-6 qubit circuits. On Clifford+T circuits, where all gates are discrete and no post-processing is possible, the model learns valid syntax and accurate T-Count statistics, yet exact equivalence degrades sharply with target length -- from 88% on circuits with <=9 gates to near zero beyond 26 gates. We trace this failure to autoregressive drift: early-token divergence cascading irrecoverably through left-to-right decoding. Two levers partially mitigate the drift: inference-time strategies that generate multiple candidates and select via equivalence verification raise exact-match rates from 7% to 22.5%, while scaling training data by 2.5x pushes them to 39.5%. Yet the degradation with target length persists -- even with more data, exact equivalence drops from 94% on short circuits to under 4% beyond 26 gates. The contrast between settings is our central finding: when approximate outputs can be rescued by post-processing, the transformer succeeds; when exact discrete correctness is required, autoregressive drift limits reliability, with both inference-time search and data scaling as effective levers while training-side fine-tuning and model-level diversification are not.