We propose a bipartite quantum key distribution (QKD) protocol based on causal nonseparability: the presence of a resource -- a process matrix -- that does not correspond to any definite causal order between two parties. In our protocol, Alice and Bob perform local operations arranged in a ``causal-order guessing game,'' whereby each round yields an 85.35\% probability of matching bits when the communication is undisturbed. This raw matching probability (or equivalently, a $\sim14.65\%$ error rate) is amenable to standard forward error-correction strategies. We further discuss the practical construction of the QKD protocol using indefinite causal order, where several different scenarios are deeply analyzed.

翻译：我们提出了一种基于因果非可分性的双向量子密钥分发协议：该协议利用一种资源——过程矩阵——该资源不对应于双方之间任何确定的因果顺序。在我们的协议中，Alice和Bob执行一种"因果序猜测游戏"中的局域操作，当通信未受干扰时，每轮操作产生85.35%的比特匹配概率。这种原始匹配概率（或等效地约14.65%的错误率）适用于标准的前向纠错策略。我们进一步讨论了利用非定域因果序构建该量子密钥分发协议的实施方案，并对多种不同场景进行了深入分析。