Operational axioms for diagonalizing states

TL;DR

This paper establishes a set of operational axioms within general probabilistic theories that guarantee the diagonalization of states, enabling a foundational understanding of majorization and entropy in quantum and generalized theories.

Contribution

It introduces four axioms that ensure state diagonalization in probabilistic theories and provides a constructive algorithm for diagonalization, linking to resource theories of purity.

Findings

A set of four axioms guarantees state diagonalization.

A constructive algorithm for diagonalizing states is developed.

A majorization criterion for state convertibility is formulated.

Abstract

In quantum theory every state can be diagonalized, i.e. decomposed as a convex combination of perfectly distinguishable pure states. This elementary structure plays an ubiquitous role in quantum mechanics, quantum information theory, and quantum statistical mechanics, where it provides the foundation for the notions of majorization and entropy. A natural question then arises: can we reconstruct these notions from purely operational axioms? We address this question in the framework of general probabilistic theories, presenting a set of axioms that guarantee that every state can be diagonalized. The first axiom is Causality, which ensures that the marginal of a bipartite state is well defined. Then, Purity Preservation states that the set of pure transformations is closed under composition. The third axiom is Purification, which allows to assign a pure state to the composition of a system with its environment. Finally, we introduce the axiom of Pure Sharpness, stating that for every system there exists at least one pure effect occurring with unit probability on some state. For theories satisfying our four axioms, we show a constructive algorithm for diagonalizing every given state. The diagonalization result allows us to formulate a majorization criterion that captures the convertibility of states in the operational resource theory of purity, where random reversible transformations are regarded as free operations.