Symmetry, Compact Closure and Dagger Compactness for Categories of Convex Operational Models

TL;DR

This paper explores the categorical structures underlying quantum and classical theories, characterizing when categories of convex operational models are compact closed or dagger compact, based on teleportation and state-conditioning principles.

Contribution

It provides new characterizations of compact closure and dagger compactness in categories of convex operational models, linking these to operational protocols like teleportation and state conditioning.

Findings

Categories are compact closed if they support teleportation protocols.

In many cases, categories are degenerate with objects being their own duals.

Dagger-compactness relates to the existence of symmetric bipartite states with isomorphic conditioning maps.

Abstract

In the categorical approach to the foundations of quantum theory, one begins with a symmetric monoidal category, the objects of which represent physical systems, and the morphisms of which represent physical processes. Usually, this category is taken to be at least compact closed, and more often, dagger compact, enforcing a certain self-duality, whereby preparation processes (roughly, states) are inter-convertible with processes of registration (roughly, measurement outcomes). This is in contrast to the more concrete "operational" approach, in which the states and measurement outcomes associated with a physical system are represented in terms of what we here call a "convex operational model": a certain dual pair of ordered linear spaces -- generally, {\em not} isomorphic to one another. On the other hand, state spaces for which there is such an isomorphism, which we term {\em weakly self-dual}, play an important role in reconstructions of various quantum-information theoretic protocols, including teleportation and ensemble steering. In this paper, we characterize compact closure of symmetric monoidal categories of convex operational models in two ways: as a statement about the existence of teleportation protocols, and as the principle that every process allowed by that theory can be realized as an instance of a remote evaluation protocol --- hence, as a form of classical probabilistic conditioning. In a large class of cases, which includes both the classical and quantum cases, the relevant compact closed categories are degenerate, in the weak sense that every object is its own dual. We characterize the dagger-compactness of such a category (with respect to the natural adjoint) in terms of the existence, for each system, of a {\em symmetric} bipartite state, the associated conditioning map of which is an isomorphism.