Quantum observables in classical frameworks

Almon, Ricardo

This work develops a quantitative framework for observer-dependent classicality in quantum mechanics based on three operational functionals: Redundancy (R), Accessibility (A), and Observer Flow (F). The A/R/F framework distinguishes environmental redundancy from the effective redundancy usable by a specific observer, models accessibility through a noisy-threshold aperture function, and introduces a continuity measure governing an observer’s capacity to integrate records over time. Classicality becomes a decomposition-indexed property, not a global feature of the universal wavefunction. Applied to Wigner’s Friend and Frauchiger–Renner, the framework dissolves paradoxes by showing that cross-agent inferences fail precisely in regimes where effective accessibility is low. The paper outlines experimental signatures, including observer-dependent classicality thresholds and flow-disruption tests, offering a route toward empirical constraints on observer-relative quantum structure.

Hans F. de Groote

Enrico G. Beltrametti, Slawomir Bugajski

The experimentally tested occurrence of probabilities that do not find a representation within the usual Kolmogorov probability theory is mathematically formalized through the notion of the Bell phenomenon for probability measures and for observables. Reference is made to a physically natural definition of observables that includes both the classical and the quantum usual versions. It is shown that even a classical framework can host the Bell phenomenon, provided fuzzy observables are called into play.

Kevin Costello, Owen Gwilliam

Zitian Zhu, Kiera Salice, Akram Touil +9 more

The transition from quantum to classical behavior is a central question in modern physics. How can we rationalize everyday classical observations from an inherently quantum world? Quantum Darwinism offers a compelling framework to explain this by proposing that the environment redundantly encodes information about a quantum system, leading to the objective reality. Here, by leveraging cutting-edge superconducting quantum circuits, we observe the highly structured branching quantum states that support classicality and the saturation of quantum mutual information, establishing a robust verification of quantum Darwinism and the underlying geometric structure of quantum states. Additionally, we propose a particular class of observables that can be used as a computationally and experimentally inexpensive quantifier to probe quantum-to-classical transitions. Our investigation delves into how the quantum effects are inaccessible to observers, allowing only classical properties to be detected. It experimentally demonstrates the physical framework through which everyday classical observations emerge from underlying quantum principles and paves the way to settling the measurement problem.