The quantum data processing inequality asserts that two quantum states become harder to distinguish when a noisy channel is applied. On the other hand, a reverse quantum data processing inequality characterizes whether distinguishability is preserved after the application of a noisy channel. In this work, we explore these concepts through contraction and expansion coefficients of the relative entropy of quantum channels. Our first result is that quantum channels with an input dimension greater than or equal to the output dimension do not have a non-zero expansion coefficient, which means that they cannot admit a reverse data-processing inequality. We propose a comparative approach by introducing a relative expansion coefficient, to assess how one channel expands relative entropy compared to another. We show that this relative expansion coefficient is positive for three important classes of quantum channels: depolarizing channels, generalized dephasing channels, and amplitude damping channels. As an application, we give the first rigorous construction of level-1 less noisy quantum channels that are non-degradable.

Quantum channel capacities give the fundamental performance limits for information flow over a communication channel. However, the prevalence of superadditivity is a major obstacle to understanding capacities, both quantitatively and conceptually. Examples of additivity, while rare, provide key insight into the origins of nonadditivity and enable our best upper bounds on capacities. Degradable channels, which have additive coherent information, are some of the only channels for which we can calculate the quantum capacity. In this paper, we introduce two families of non-degradable channels whose coherent information remains additive, making their quantum capacities tractable. First, we demonstrate that channels capable of “outperforming” their environment, under conditions weaker than degradability, can exhibit either strong or weak additivity of coherent information. Second, we explore a complementary construction that modifies a channel to preserve coherent information additivity while destroying the “outperforming” property. We analyze how structural constraints guarantee strong and weak additivity and investigate how relaxing these constraints leads to the failure of strong additivity, with weak additivity potentially persisting.

Quantum capacity exhibits intriguing phenomena such as super-activation, amplification, and super-amplification, which can be explored using constructions based on private channels. In this work, we refine the connection between private channels and additivity questions in quantum channel capacity. Specifically, the shield system, which safeguards the key system's privacy by leaking quantum information, can recover some of this lost information when paired with an auxiliary channel like the erasure channel$\mathcal{E}_{p}$, thereby enhancing overall capacity. Building on this insight, we construct new and simple examples of super-amplification for the erasure channel$\mathcal{E}_{p}$with any erasure probability$p$. In addition, we provide a refined version of the Smith-Yard argument that also applies for arbitrary$p$. Finally, we revisit the fundamental question of whether regularization in channel capacity formulas requires infinitely many channel uses, showing that the necessary number may instead depend on the channel's dimension.

Quantum channel capacities give the fundamental performance limits for information flow over a communication channel. However, the prevalence of superadditivity is a major obstacle to understanding capacities, both quantitatively and conceptually. In contrast, examples exhibiting additivity, though relatively rare, offer crucial insights into the origins of nonadditivity and form the basis of our strongest upper bounds on capacity. Degradable channels, whose coherent information is provably additive, stand out as among the few classes of channels for which the quantum capacity is exactly computable. In this paper, we introduce two families of non-degradable channels whose coherent information remains additive, making their quantum capacities tractable. First, we demonstrate that channels capable of “outperforming” their environment, under conditions weaker than degradability, can exhibit either strong or weak additivity of coherent information. Second, we explore a complementary construction that modifies a channel to preserve coherent information additivity while destroying the “outperforming” property. We analyze how structural constraints guarantee strong and weak additivity and investigate how relaxing these constraints leads to the failure of strong additivity, with weak additivity potentially persisting.

We develop a connection between tripartite information I3, quantum secret sharing protocols and multi-unitaries. This leads to a general framework of constructing ((2, 3)) threshold schemes in arbitrary dimension. As an application, we propose a class of random codes generated by Haar-distributed random unitaries, in which all states have bounded tripartite information with high probability. Moreover, using the I3criteria for imperfect sharing schemes, we discover examples of VIP secret sharing schemes.