Optimal control with flag qubits
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arXiv
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| Format: | Preprint |
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2026
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| _version_ | 1866917335755915264 |
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| author | Xie, Liang-Xu de Paula, Lui Zuccherelli Cai, Weizhou Jie, Qing-Xuan Sun, Luyan Zou, Chang-Ling Guo, Guang-Can Chen, Zi-Jie Zou, Xu-Bo |
| author_facet | Xie, Liang-Xu de Paula, Lui Zuccherelli Cai, Weizhou Jie, Qing-Xuan Sun, Luyan Zou, Chang-Ling Guo, Guang-Can Chen, Zi-Jie Zou, Xu-Bo |
| contents | High-fidelity quantum operations are the cornerstone of fault-tolerant quantum computation. In open quantum systems, traditional optimal control only passively resists decoherence, leaving environment-induced uncertainty as a fundamental performance bottleneck. To overcome this, we propose a new optimal control framework with flag ancillas and the Flag-GRAPE algorithm, which can actively tailor the system's noise structure. Through embedding post-selection directly into the objective function, Flag-GRAPE correlates decoherence errors with the ancilla's unexpected state. Subsequent measurement and post-selection effectively expel this uncertainty, circumventing the fidelity bounds of traditional control. Numerical simulations in a superconducting quantum circuit demonstrate a $51\%$ reduction in infidelity compared to traditional closed-system pulses and also show that such enhancement is robust across broad noise regimes. Furthermore, by actively converting unstructured decoherence into heralded erasure errors, Flag-GRAPE is inherently compatible with quantum error correction. We demonstrate this by initializing a logical cat-code state, showing that the combination between Flag-GRAPE and QEC yields immediate state preparation enhancements. This new framework can reduce hardware overhead for fault-tolerant architectures and open up a practical path toward logical state preparation gain in near-term experiments. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_12162 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Optimal control with flag qubits Xie, Liang-Xu de Paula, Lui Zuccherelli Cai, Weizhou Jie, Qing-Xuan Sun, Luyan Zou, Chang-Ling Guo, Guang-Can Chen, Zi-Jie Zou, Xu-Bo Quantum Physics High-fidelity quantum operations are the cornerstone of fault-tolerant quantum computation. In open quantum systems, traditional optimal control only passively resists decoherence, leaving environment-induced uncertainty as a fundamental performance bottleneck. To overcome this, we propose a new optimal control framework with flag ancillas and the Flag-GRAPE algorithm, which can actively tailor the system's noise structure. Through embedding post-selection directly into the objective function, Flag-GRAPE correlates decoherence errors with the ancilla's unexpected state. Subsequent measurement and post-selection effectively expel this uncertainty, circumventing the fidelity bounds of traditional control. Numerical simulations in a superconducting quantum circuit demonstrate a $51\%$ reduction in infidelity compared to traditional closed-system pulses and also show that such enhancement is robust across broad noise regimes. Furthermore, by actively converting unstructured decoherence into heralded erasure errors, Flag-GRAPE is inherently compatible with quantum error correction. We demonstrate this by initializing a logical cat-code state, showing that the combination between Flag-GRAPE and QEC yields immediate state preparation enhancements. This new framework can reduce hardware overhead for fault-tolerant architectures and open up a practical path toward logical state preparation gain in near-term experiments. |
| title | Optimal control with flag qubits |
| topic | Quantum Physics |
| url | https://arxiv.org/abs/2603.12162 |