As an important branch of quantum communication, quantum secure direct communication (QSDC) can directly transmit secure information between two communication parties without keys. Measurement-device-independent quantum secure direct communication (MDI–QSDC) is immune to all the attacks at detector’s side and thus can eliminate the security loopholes associated with measurement devices. Existing MDI–QSDC protocols encode secure information in only one degree of freedom (DOF) of photons. In this paper, we propose a high-capacity MDI–QSDC protocol by encoding secure information in photons’ polarization and spatial-mode DOFs. With hyper-encoding in two DOFs, we can double photons’ channel capacity and effectively increase MDI–QSDC’s communication efficiency. Moreover, for each encoded qudit, the states in two DOFs are totally independent. If a detection failure or error occurs on one DOF, the state in the other DOF can still be used to transmit secure information. This hyper-encoding MDI–QSDC may have potential applications in future quantum communication.

作为量子通信的重要分支，量子安全直接通信（QSDC）可以在没有密钥的情况下在两个通信方之间直接传输安全信息。独立于测量设备的量子安全直接通信（MDI-QSDC）不受检测器侧的所有攻击的侵害，因此可以消除与测量设备相关的安全漏洞。现有的MDI-QSDC协议仅在光子的一个自由度（DOF）中编码安全信息。在本文中，我们通过在光子的极化和空间模式自由度中编码安全信息，提出了一种大容量的MDI-QSDC协议。通过在两个自由度中进行超编码，我们可以使光子的通道容量增加一倍，并有效提高MDI-QSDC的通信效率。而且，对于每个编码的qudit，两个自由度中的状态是完全独立的。如果一个DOF上发生检测失败或错误，则另一个DOF中的状态仍可以用于传输安全信息。这种超编码MDI-QSDC可能在未来的量子通信中具有潜在的应用。