Quantum secure direct communication (QSDC) is capable of direct confidential communications over a quantum channel, which is achieved by dispensing with the key agreement channel of the well-known quantum key distribution (QKD). However, to make QSDC a practical reality, we have to mitigate its reliance on quantum memory, its immediate communication interruption caused by eavesdropping and its low transmission reliability due to the heavy qubit losses. Hence a new QSDC protocol is proposed based on a sophisticated coded single-photon DL04 QSDC protocol to tackle the open challenges. In particular, quantum memory is dispensed with and a high-accuracy secrecy capacity estimate is derived for this protocol by conceiving dynamic joint encryption and error-control (JEEC) coding. We demonstrate that this quantum-memory-free DL04 QSDC (QMF-DL04 QSDC) protocol inches closer to the quantum channel’s capacity and significantly improves the original DL04 QSDC’s robustness. Moreover, a rate-compatible low-rate JEEC coding scheme is designed for the proposed framework, and the JEEC code advocated is shown to approach the secrecy capacity, despite tolerating an extremely high loss of qubits in the time-varying wiretap channel. Our simulations and experimental results demonstrate that the QMF-DL04 QSDC scheme significantly increases both the secure information rate and the communication distance of the original DL04 protocol.

中文翻译：

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迈向实用量子安全直接通信：无量子内存协议和代码设计

量子安全直接通信（QSDC）能够通过量子通道进行直接的机密通信，这是通过省去众所周知的量子密钥分发（QKD）的密钥协商通道来实现的。然而，要使 QSDC 成为现实，我们必须减轻其对量子存储器的依赖、由于窃听导致的即时通信中断以及由于大量量子比特损失导致的低传输可靠性。因此，基于复杂的编码单光子 DL04 QSDC 协议提出了一种新的 QSDC 协议，以应对开放挑战。特别是，通过构想动态联合加密和错误控制 (JEEC) 编码，省去了量子存储器，并为该协议导出了高精度的保密容量估计。我们证明了这种无量子内存的 DL04 QSDC (QMF-DL04 QSDC) 协议更接近量子通道的容量，并显着提高了原始 DL04 QSDC 的稳健性。此外，为所提出的框架设计了一种速率兼容的低速率 JEEC 编码方案，尽管在随时间变化的窃听通道中可以容忍极高的量子比特损失，但所倡导的 JEEC 代码仍接近保密容量。我们的模拟和实验结果表明，QMF-DL04 QSDC 方案显着提高了原始 DL04 协议的安全信息速率和通信距离。为所提出的框架设计了一种速率兼容的低速率 JEEC 编码方案，尽管在时变窃听通道中可以容忍极高的量子比特损失，但所倡导的 JEEC 代码仍接近保密容量。我们的模拟和实验结果表明，QMF-DL04 QSDC 方案显着提高了原始 DL04 协议的安全信息速率和通信距离。为所提出的框架设计了一种速率兼容的低速率 JEEC 编码方案，尽管在时变窃听通道中可以容忍极高的量子比特损失，但所倡导的 JEEC 代码仍接近保密容量。我们的模拟和实验结果表明，QMF-DL04 QSDC 方案显着提高了原始 DL04 协议的安全信息速率和通信距离。