Brandão's generalised quantum Stein's lemma

Let (Mn)n(\mathcal{M}_n)_n be a family of sets of quantum states satisfying Axioms 1--5 in Section~, and let DM(ρ)D^\infty_{\mathcal{M}}(\rho) denote the corresponding regularised relative entropy. Brandão's generalised quantum Stein's lemma. For every quantum state ρ\rho,

limϵ0limn1nminσnMnDHϵ(ρnσn)=DM(ρ).\lim_{\epsilon \to 0} \lim_{n \to \infty} \frac{1}{n} \min_{\sigma_n \in \mathcal{M}_n} D^\epsilon_H\left(\rho^{\otimes n} \middle\| \sigma_n\right)=D^\infty_{\mathcal{M}}(\rho).

This extends quantum Stein's lemma from a single i.i.d. alternative state to composite hypothesis testing against families of states, and connects the asymptotic hypothesis-testing exponent with the regularised relative entropy. The supplied text presents it as the seminal result of Brandão and does not provide evidence resolving the candidate's status.

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Primary source

Mario Berta, Fernando G. S. L. Brandão, Gilad Gour, Ludovico Lami, Martin B. Plenio, Bartosz Regula and Marco Tomamichel, “On a gap in the proof of the generalised quantum Stein's lemma and its consequences for the reversibility of quantum resources”, arXiv:2205.02813 (2025).

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