A tight uniform continuity bound for the Arimoto-Rényi conditional entropy and its extension to classical-quantum states

Abstract

We prove a tight uniform continuity bound for Arimoto's version of the conditional $\alpha$-Rényi entropy, for the range $\alpha \in [0,1)$. This definition of the conditional Rényi entropy is the most natural one among the multiple forms which exist in the literature, since it satisfies two desirable properties of a conditional entropy, namely, the fact that conditioning reduces entropy, and that the associated reduction in uncertainty cannot exceed the information gained by conditioning. Furthermore, it has found interesting applications in various information theoretic tasks such as guessing with side information and sequential decoding. This conditional entropy reduces to the conditional Shannon entropy in the limit $\alpha \to 1$, and this in turn allows us to recover the recently obtained tight uniform continuity bound for the latter from our result. Finally, we apply our result to obtain a tight uniform continuity bound for the conditional $\alpha$-Rényi entropy of a classical-quantum state, for $\alpha$ in the same range as above. This again yields the corresponding known bound for the conditional entropy of the state in the limit $\alpha \to 1$.

23 pages. Changes in v2: new references added and minor corrections to existing references. Changes in v3: last version as published, minor corrections, Appendix 3 added with figures in order to illustrate the steps of the proof of Theorem 1

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• A Sharp Fannes-type Inequality for the von Neumann Entropy
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Cited by in corpus (3)

• Continuous majorization in quantum phase space
• Operator-valued Schatten spaces and quantum entropies
• Lipschitz continuity of quantum-classical conditional entropies with respect to angular distance and related properties