A single-photon entangled state (or single-particle entangled state (SPES) in general) can offer a more secure way of encoding and processing quantum information than their multi-photon (or multi-particle) counterparts. The SPES generated via a 2D alternate quantum-walk setup from initially separable states can be either 3-way or 2-way entangled. This letter shows that the generated genuine three-way and nonlocal two-way SPES can be used as cryptographic keys to securely encode two distinct messages simultaneously. We detail the message encryption-decryption steps and show the resilience of the 3-way and 2-way SPES-based cryptographic protocols against eavesdropper attacks like intercept-and-resend and man-in-the-middle. We also detail the experimental realization of these protocols using a single photon, with the three degrees of freedom being OAM, path, and polarization. We have proved that the protocols have unconditional security for quantum communication tasks. The ability to simultaneously encode two distinct messages using the generated SPES showcases the versatility and efficiency of the proposed cryptographic protocol. This capability could significantly improve the throughput of quantum communication systems.

翻译：单光子纠缠态（或广义上的单粒子纠缠态）相比多光子（或多粒子）纠缠态，能够为量子信息的编码与处理提供更安全的方式。通过二维交替量子行走装置从初始可分离态生成的单光子纠缠态，可以是三体或二体纠缠态。本文证明，所生成的真三体与非局域二体单光子纠缠态可作为密码密钥，同时安全地编码两条独立消息。我们详细阐述了消息的加密-解密步骤，并展示了基于三体与二体单光子纠缠态的密码协议对截获-重发与中间人等窃听攻击的抵抗能力。同时，我们详述了利用单光子的三个自由度（轨道角动量、路径与偏振）实现这些协议的实验方案。我们已证明该协议在量子通信任务中具有无条件安全性。利用生成的单光子纠缠态同时编码两条独立消息的能力，体现了所提出密码协议的多功能性与高效性。这一特性有望显著提升量子通信系统的吞吐量。