Infinite dimensionality of the post-processing order of measurements on a general state space

TL;DR

This paper proves that the order and order monotone dimensions of the post-processing order of measurements are infinite for general state spaces and quantum channels, resolving an open question in quantum measurement theory.

Contribution

It establishes the infinite dimensionality of the post-processing order in generalized probabilistic theories and quantum systems, providing a negative answer to an open problem.

Findings

Order and order monotone dimensions are infinite for non-singleton state spaces.

Quantum post-processing relations have countably infinite order dimensions when the Hilbert space dimension is at least 2.

Results apply to a broad class of probabilistic theories, including quantum mechanics.

Abstract

For a partially ordered set $(S, \mathord\preceq)$, the order (monotone) dimension is the minimum cardinality of total orders (respectively, real-valued order monotone functions) on $S$ that characterize the order $\preceq$. In this paper we consider an arbitrary generalized probabilistic theory and the set of finite-outcome measurements on it, which can be described by effect-valued measures, equipped with the classical post-processing orders. We prove that the order and order monotone dimensions of the post-processing order are (countably) infinite if the state space is not a singleton (and is separable in the norm topology). This result gives a negative answer to the open question for quantum measurements posed in [Guff T \textit{et al.\/} 2021 \textit{J.\ Phys.\ A: Math.\ Theor.} \textbf{54} 225301]. We also consider the quantum post-processing relation of channels with a fixed input quantum system described by a separable Hilbert space $\mathcal{H}$ and show that the order (monotone) dimension is countably infinite when $\dim \mathcal{H} \geq 2$.