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Quantum reference frame transformations as symmetries and the paradox of the third particle
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In a quantum world, reference frames are ultimately quantum systems too -- but what does it mean to "jump into the perspective of a quantum particle"? In this work, we show that quantum reference frame (QRF) transformations appear naturally as symmetries of simple physical systems. This allows us to rederive and generalize known QRF transformations within an alternative, operationally transparent framework, and to shed new light on their structure and interpretation. We give an explicit description of the observables that are measurable by agents constrained by such quantum symmetries, and apply our results to a puzzle known as the `paradox of the third particle'. We argue that it can be reduced to the question of how to relationally embed fewer into more particles, and give a thorough physical and algebraic analysis of this question. This leads us to a generalization of the partial trace (`relational trace') which arguably resolves the paradox, and it uncovers important structures of constraint quantization within a simple quantum information setting, such as relational observables which are key in this resolution. While we restrict our attention to finite Abelian groups for transparency and mathematical rigor, the intuitive physical appeal of our results makes us expect that they remain valid in more general situations.
Forward citations
Cited by 6 Pith papers
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For closed quantum N-particle systems all 3N canonical degrees of freedom are physical; the frame degrees of freedom that relational models discard reappear as non-Heisenberg terms in generalised uncertainty relations...
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Quantum reference frames built from Wilson lines give lattice gauge theories gauge-invariant subsystem factorizations and a hierarchy of relational entanglement entropies.
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For ideal quantum reference frames with a single constraint, the perspective-neutral, algebraic, and effective semiclassical approaches describe the same physics and the same frame-switching rules.
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Gravitational subregion entropy is observer-dependent: different quantum clocks produce different von Neumann algebras and different entropy functionals for the same global state.
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Position-superposed labs define quantum reference frames operationally, differ from Wigner's-friend observers, and can broadcast outcomes without decohering their position superposition.
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A two-observer Z2 toy model is used to argue that internal observers can access the total charge, favoring weak over strong symmetry in quantum reference frame frameworks.
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