Categorical characterization of the Petz recovery map

Let R\mathsf{R} be a retrodiction, or quantum Bayesian inversion, assignment for normal quantum processes between von Neumann algebras equipped with faithful states, satisfying the proposed categorical axioms for retrodiction. Is R\mathsf{R} necessarily equal, for every such process and faithful state, to the corresponding Petz recovery map P\mathsf{P}? Equivalently, do the categorical axioms uniquely characterize the Petz recovery map in the infinite-dimensional von Neumann-algebraic setting, so that R=P\mathsf{R}=\mathsf{P}?

Progress summary

Partially solved

A new formulation sharpens the infinite-dimensional question but does not settle whether the Petz recovery map is uniquely characterized.

The problem asks whether a proposed structural description of quantum Bayesian inversion uniquely identifies the Petz recovery map in the infinite-dimensional setting. No proposer or original date is identified in the available record.

August 2026 formalization

The question has been formalized with a detailed framework for faithful states on von Neumann algebras, clarifying its mathematical scope. The source explicitly makes no claim to resolve the uniqueness question.

Current status (as of August 2026): The infinite-dimensional problem is more precisely formulated, but uniqueness of the categorical characterization remains open.

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