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

Abstract

For a partially ordered set (S, ⪯), the order (monotone) dimension is the minimum cardinality of total orders (respectively, real-valued order monotone functions) on S that characterize the order ⪯. 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 et al 2021 J. Phys. A: Math. Theor. 54 225301). We also consider the quantum post-processing relation of channels with a fixed input quantum system described by a separable Hilbert space and show that the order (monotone) dimension is countably infinite when .