Bidirectional quantum key distribution (QKD) protocols face persistent challenges related to classical disclosure, confinement of the signal space to predictable subspaces, and limited detectability under substitution or entanglement-swapping attacks. In this work, we present a Bell-state extension of the Loop-Back QKD architecture that improves efficiency and detectability while preserving its defining feature of a simplified, measurement-free remote terminal. The protocol employs entangled Bell states together with deterministic local Pauli encoding at the remote node. A central element is that Alice privately prepares and knows the initial Bell state, which serves as a hidden reference enabling her to interpret the Bell-state transition induced by Bob, while preventing an adversary from reconstructing the encoding without access to this reference. By exploiting both intra- and inter-family Bell transitions, the scheme expands the effective signal space beyond the subspace restrictions of earlier two-way protocols. Alice performs a Bell-state measurement to deterministically infer Bob's operation without any basis sifting. Although the traveling subsystem remains locally maximally mixed, concealing the initial Bell family amplifies disturbance under separable substitution strategies, yielding an intrinsic detection probability of approximately 3/4 per round. From an efficiency perspective, the protocol lifts the intrinsic post-selection limitation of single-qubit Loop-Back schemes: the effective throughput is bounded only by the Bell-state measurement success probability, reaching up to 50% in linear-optical implementations. These features make the proposed scheme particularly suitable for mobile or edge-based QKD scenarios, where passive remote nodes must operate under high loss and limited interaction times.

翻译：双向量子密钥分发协议面临一系列持续挑战，包括经典信息泄露、信号空间受限至可预测子空间，以及替换攻击或纠缠交换攻击下可检测性有限。本文提出一种基于贝尔态扩展的回环量子密钥分发架构，在保留简化且免测量远端终端这一核心特征的同时，提升了效率与可检测性。该协议使用纠缠贝尔态，并允许远端节点执行确定性的局域泡利编码。其核心要素在于：发送方Alice私下制备并知晓初始贝尔态，该贝尔态充当隐藏参考基准，使其能够解读Bob引发的贝尔态跃迁，同时防止攻击者在无法获取此参考基准的情况下重构编码。通过利用族内和族间贝尔态跃迁，该方案将有效信号空间拓展至早期双向协议子空间限制之外。Alice执行贝尔态测量以确定性推断Bob的操作，无需任何基矢筛选。尽管传输子系统在局部保持最大混合态，隐藏初始贝尔族会放大可分态替换策略下的扰动，从而实现每轮约3/4的固有检测概率。从效率角度而言，该协议突破了单量子比特回环方案固有的后选择限制：有效吞吐量仅受限于贝尔态测量成功概率，在线性光学实现中可达50%。这些特性使所提方案特别适用于需要高损耗及有限交互时间条件下被动远端节点运行的移动或边缘量子密钥分发场景。