The one-way distillable entanglement is a central operational measure of bipartite entanglement, quantifying the optimal rate at which maximally entangled pairs can be extracted by one-way LOCC. Despite its importance, it is notoriously hard to compute, since it is defined by a regularized optimization over many copies and adaptive one-way protocols. At present, single-letter formulas are only known for (conjugate) degradable and PPT states. More generally, it has remained unclear when one-way distillable entanglement can still be additive beyond degradability and PPT settings, and how such additivity relates to additivity questions of quantum capacity of channels. In this paper, we address this gap by identifying three explicit families of non-degradable and non-PPT states whose one-way distillable entanglement is nevertheless single-letter. First, we introduce two weakened degradability-type conditions--regularized less-noisy and informationally degradable--and prove that each guarantees additivity and hence a single-letter formula. Second, we show a stability result for orthogonally flagged mixtures: when one component has orthogonal support on Alice's system and zero one-way distillable entanglement, the mixture remains single-letter, even though degradability is typically lost under such mixing. Finally, we propose a generalized spin-alignment principle for entropy minimization in tensor-product settings, which we establish in several key cases, including a complete Rényi-2 result. As an application, we obtain additivity results for generalized direct-sum channels and their corresponding Choi states.

翻译：单向可蒸馏纠缠是双粒子纠缠的一种核心操作性度量，它量化了通过单向LOCC提取最大纠缠对的最优速率。尽管其重要性不言而喻，但由于它涉及对多副本和自适应单向协议的规则化优化，计算起来极为困难。目前，仅对（共轭）退化和PPT态已知其单字母公式。更一般地，人们仍不清楚在退化性和PPT情形之外，单向可蒸馏纠缠何时仍具有可加性，以及这种可加性与信道量子容量的可加性问题有何关联。本文通过识别三个非退化且非PPT态族填补了这一空白，这些态的单向可蒸馏纠缠仍具有单字母形式。首先，我们引入了两种弱化退化性条件——正则化更少噪声和信息可退化——并证明每个条件都保证了可加性，从而得到单字母公式。其次，我们展示了正交标记混合态的稳定性结果：当一个分量在Alice系统上具有正交支撑且单向可蒸馏纠缠为零时，即使混合通常会导致退化性丢失，该混合态仍保持单字母性质。最后，我们提出了张量积设置中熵最小化的广义自旋对齐原理，并在若干关键情形中（包括完整的Rényi-2结果）建立了该原理。作为应用，我们获得了广义直和信道及其对应Choi态的可加性结果。