We report an experimental implementation of free-space quantum secure direct communication based on single photons. The quantum communication scheme uses phase encoding, and the asymmetric Mach-Zehnder interferometer is optimized so as to automatically compensate phase drift of the photons during their transitions over the free-space medium. An information transmission rate of 500 bps over a 10-meter free space with a mean quantum bit error rate of 0.49%$\pm$0.27% is achieved. The security is analyzed under the scenario that Eve performs collective attack and photon number splitting collective attack. Our results show that quantum secure direct communication is feasible in free space.

中文翻译：

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实验性自由空间量子安全直接通信及其安全分析

我们报告了基于单光子的自由空间量子安全直接通信的实验实现。量子通信方案使用相位编码，并且对非对称马赫-曾德干涉仪进行了优化，以自动补偿光子在自由空间介质上跃迁期间的相位漂移。在 10 米自由空间上实现了 500 bps 的信息传输速率，平均量子比特误码率为 0.49%$\pm$0.27%。安全性分析在Eve进行集体攻击和光子数分裂集体攻击的场景下。我们的结果表明，量子安全直接通信在自由空间中是可行的。