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arXiv · 2604.09772

Leggett-Garg Inequality Violations Bound Quantum Fisher Information

Nick Abboud·Yuntao Guan·Barry Bradlyn·Jorge Noronha
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Abstract

We prove that a violation of a Leggett-Garg inequality for bounded observables in stationary pure states and thermal states yields a rigorous lower bound on the quantum Fisher information. This turns a qualitative foundations test of realism in quantum systems into a quantitative witness of useful quantum sensitivity and, in the collective setting, into a lower bound on multipartite entanglement depth in many-body systems. We further demonstrate that Leggett-Garg violations are constrained by the same spectral moments, susceptibilities, and $f$-sum-rule bounds that organize many-body response. Our results show that temporal correlations of a single collective observable can serve as an experimentally accessible witness of many-body quantum coherence, without requiring full state reconstruction.

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Nick Abboud, Yuntao Guan, Barry Bradlyn, Jorge Noronha. 2026-04-10. Leggett-Garg Inequality Violations Bound Quantum Fisher Information. https://arxiv.org/abs/2604.09772

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