We propose a free-space quantum secure direct communication protocol by using hyper-encoded single photons. The communication parties can securely exchange private information in a deterministic way. In this protocol, single photons are encoded simultaneously by both the polarization and spatial-mode degrees of freedom that constitute a decoherence-free space. The detrimental effect introduced by either reference-frame misalignment or practical channel rotational noise is passively suppressed. Finally, the security of our protocol against classical individual attacks is proved by quantum mutual information.

中文翻译：

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基于无相干空间的自由空间量子安全直接通信

我们提出了使用超编码单光子的自由空间量子安全直接通信协议。通信方可以确定性方式安全地交换私人信息。在该协议中，单个光子同时通过偏振和空间模式自由度进行编码，这些自由度构成了无相干空间。由参考帧未对准或实际通道旋转噪声引入的有害影响被被动抑制。最后，量子互信息证明了我们的协议针对经典个体攻击的安全性。