Against probability: A quantum state is more than a list of probability distributions

Ladina Hausmann, Renato Renner (Jan 28 2026).

Abstract: The state $\rho$ of a quantum system can be represented by a vector $\mathbf{P}_{\mathcal{M}}(\rho)$ of outcome probabilities for a set of measurements $\mathcal{M}$. Such representations appear throughout physics, for example, in quantum field theory via correlation functions and in quantum foundations within generalized probabilistic frameworks. In this work, we identify an unavoidable tension: to enable operationally meaningful statements, the map ${\rho \mapsto \mathbf{P}_{\mathcal{M}}(\rho)}$ must be topologically robust $\unicode{x2013}$ preserving the notion of closeness between states. Yet, a probability representation that is topologically robust cannot simultaneously retain other essential structure, such as the subsystem structure.

Arxiv: https://arxiv.org/abs/2601.18872