Quantum secure direct communication (QSDC) transmits information directly over a quantum channel. In addition to security in transmission, it avoids loopholes of key loss and prevents the eavesdropper from getting ciphertext. In this article, we propose a QSDC protocol using entangled photon pairs. This protocol differs from existing entanglement-based QSDC protocols because it does not perform Bell-state measurement, and one photon of the entangled pair is measured after the entanglement distribution. It has the advantage of high signal-to-noise ratio due to the heralding function of entanglement pairs, and it also has the relative ease in performing single-photon measurement. The protocol can use a practical entanglement source from spontaneous parametric down-conversion (SPDC); Gottesman-Lo-Lütkenhaus-Preskill theory and the decoy state method give a better estimate of the error rate. Security analysis is completed with Wyner’s wiretap channel theory, and the lower bound of the secrecy capacity is estimated. Numerical simulations were carried out to study the performance of the protocol. These simulations demonstrated that the protocol with a practical SPDC entanglement source performed well and was close to the case with an ideal entanglement source.

中文翻译：

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与纠缠源和单光子测量的量子安全直接通信

量子安全直接通信（QSDC）直接在量子信道上传输信息。除了传输的安全性之外，它还避免了密钥丢失的漏洞并防止窃听者获得密文。在本文中，我们提出了使用纠缠光子对的QSDC协议。该协议与现有的基于纠缠的QSDC协议不同，因为它不执行贝尔状态测量，并且在纠缠分布之后测量纠缠对中的一个光子。由于具有纠缠对的预言功能，因此具有高信噪比的优势，并且在执行单光子测量时也相对容易。该协议可以使用来自自发参数下变频（SPDC）的实用纠缠源。Gottesman-Lo-Lütkenhaus-Preskill理论和诱骗状态方法可以更好地估计错误率。使用Wyner的窃听通道理论完成安全性分析，并估计保密容量的下限。进行了数值模拟以研究协议的性能。这些仿真表明，具有实用SPDC纠缠源的协议性能良好，并且与具有理想纠缠源的情况非常接近。