AI 中文总结
研究如何实现跨混合光纤和自由空间链路的端到端量子密钥分发，利用全光编码转换执行诱骗态BB84协议，在不同大气条件下实现连续安全密钥生成，验证光子级操作，确立编码转换为实用接口，为量子网络应用提供构建模块。
AI 中文摘要
量子密钥分发（QKD）有望实现信息理论上的安全通信，但未来网络必须连接自然采用不同光子编码的光纤和自由空间链路，即光纤中的时间编码和自由空间中的偏振编码。本文展示了一个完整的混合光纤和自由空间QKD链路，在单个端到端协议中连接两种介质，完全在光域内进行两种编码之间的转换。使用在1550nm运行的诱骗态BB84协议，在90m室外自由空间链路上演示了连续安全密钥生成。该系统在折射率结构参数Cn^2跨越两个数量级以上的大气条件下运行，从强烈的白天湍流到安静的夜间条件，并在750m自由空间扩展上进一步验证了光子级操作。整个链路的会话平均量子比特误码率（QBER）保持在5.6-6.8%，远低于11%的BB84安全阈值。编码转换完全在光域内进行，无需测量或状态重建，保留了BB84协议的安全假设。因此，时间编码到偏振（T2P）和偏振到时间编码（P2T）转换器仍然是不可信量子信道的一部分，而不是可信中间节点。这些结果确立了安全光子编码转换作为光纤和自由空间量子通信平台之间的实用接口，为未来量子网络应用提供了一个构建模块。
英文摘要
Quantum key distribution (QKD) promises information-theoretically secure communication, but future networks must bridge fiber and free-space links that naturally employ different photonic encodings, namely time-bin in fiber and polarization in free space. Here we demonstrate a complete hybrid fiber and free-space QKD link that bridges both media within a single end-to-end protocol, converting between the two encodings entirely in the optical domain. Using the decoy-state BB84 protocol operating at 1550 nm, we demonstrate continuous secure-key generation over a 90 m outdoor free-space link. The system operates across atmospheric conditions spanning more than two orders of magnitude in the refractive-index structure parameter Cn^2, from strong daytime turbulence to quiescent nighttime conditions, and we further validate photon-level operation over a 750 m free-space extension. Throughout, the link maintains a session-mean quantum bit error rate (QBER) of 5.6-6.8%, well below the 11% BB84 security threshold. The encoding conversion is performed entirely in the optical domain without measurement or state reconstruction, preserving the security assumptions of the BB84 protocol. Consequently, the time-bin-to-polarization (T2P) and polarization-to-time-bin (P2T) converters remain part of the untrusted quantum channel rather than trusted intermediate nodes. These results establish secure photonic encoding conversion as a practical interface between fiber and free-space quantum communication platforms, providing a building block for future quantum networks applications.
