Complete Relational Description of Spin in a Quantum Background

Abstract:The standard description of the state of a spin in quantum mechanics presupposes externally fixed directions -- a classical background. Can a spin be fully described instead in relation to other quantum mechanical systems? Poulin suggested twenty years ago group averaging over rotations the joint state of a fundamental spin and a reference spin with large angular momentum which, however, yields a classical bit in a probabilistic mixture. We revisit this idea and show that when the quantum reference system is augmented to \emph{two} large spins, the standard quantum mechanical description of a spin is recovered in the limit of large quantum numbers for the reference system.

arXiv:2606.15873 [quant-ph]

(or arXiv:2606.15873v1 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2606.15873 Focus to learn more arXiv-issued DOI via DataCite (pending registration)

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