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arXiv:2402.09713 · Proceedings of the American Mathematical Society

Variations on quantum de Finetti theorems and operator valued Martin boundaries: A Choquet–Deny approach

Benoı̂t Collins, Thierry Giordano, Ryosuke Sato

Published Jan 15, 2026• 0 citations
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Abstract

We revisit the quantum de Finetti theorem. We state and prove a couple of variants thereof. In parallel, we introduce an operator version of the Martin boundary on quantum groups and prove generalizations of Biane’s theorem. Our proof of the de Finetti theorem is new in the sense that it is based on an analogy with the theory of operator-valued Martin boundary that we introduce.

DOI
10.1090/proc/17486

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We revisit the quantum de Finetti theorem. We state and prove a couple of variants thereof. In parallel, we introduce an operator version of the Martin boundary on quantum groups and prove generalizations of Biane’s theorem. Our proof of the de Finetti theorem is new in the sense that it is based on an analogy with the theory of operator-valued Martin boundary that we introduce.
We show an organized form of quantum de Finetti theorem for Boolean independence. We define a Boolean analogue of easy quantum groups for the categories of interval partitions, which is a family of sequences of quantum semigroups. We construct the Haar states on those quantum semigroups. The proof of our de Finetti theorem is based on the analysis of the Haar states. [Modified]Definition of the Boolean quantum semigroups on categories of interval partitions [Delete]Classification of categories of interval partitions [Add]Proof of the positiveness of the Haar functionals (in particular they are Haar states)
Markov categories have recently emerged as a powerful high-level framework for probability theory and theoretical statistics. Here we study a quantum version of this concept, called involutive Markov categories. These are equivalent to Parzygnat's quantum Markov categories, but we argue that they offer a simpler and more practical approach. Our main examples of involutive Markov categories have pre-C*-algebras, including infinite-dimensional ones, as objects, together with completely positive unital maps as morphisms in the picture of interest. In this context, we prove a quantum de Finetti theorem for both the minimal and the maximal C*-tensor norms, and we develop a categorical description of such quantum de Finetti theorems which amounts to a universal property of state spaces.
The quantum de Finetti theorem asserts that the |kkk|-body density matrices of an |NNN|-body bosonic state approach a convex combination of Hartree states (pure tensor powers) when |NNN| is large and |kkk| fixed. In this note, we review a construction due to Christandl et al. [8] valid for finite-dimensional Hilbert spaces, which gives a quantitative version of the theorem. We first propose a variant of their proof that leads to a slightly improved estimate. Next we provide an alternative proof of an explicit formula due to Chiribella [7], which gives the density matrices of the constructed state as a function of those of the original state.