Complete Relational Description of Spin in a Quantum Background

• URL: http://arxiv.org/abs/2606.15873v1
• Date: Sun, 14 Jun 2026 15:46:01 GMT
• Title: Complete Relational Description of Spin in a Quantum Background
• Abstract summary: Poulin suggested twenty years ago group averaging over rotations the joint state of a fundamental spin and a reference spin with large angular momentum.<n>We show that when the quantum reference system is augmented to emphtwo large spins, the standard quantum mechanical description of a spin is recovered in the limit of large quantum numbers for the reference system.
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• License: http://creativecommons.org/licenses/by/4.0/
• 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.

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