Sharp State-Independent Uncertainty Relations for Multipartite systems

• URL: http://arxiv.org/abs/2608.22656v1
• Date: Sun, 23 Aug 2026 23:33:26 GMT
• Title: Sharp State-Independent Uncertainty Relations for Multipartite systems
• Abstract summary: State-independent uncertainty relations ask how much fluctuation remains unavoidable for every quantum state.<n>For observables generated by a continuous symmetry, sharp state-independent bounds are known when the symmetry representation is irreducible.<n>We exploit this problem for multipartite quantum systems with a compact semisimple symmetry algebra $mathfrak g$.
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• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Uncertainty relations constrain the fluctuations of incompatible observables, but most familiar bounds depend on the quantum state. State-independent uncertainty relations instead ask how much fluctuation remains unavoidable for every quantum state. For observables generated by a continuous symmetry, sharp state-independent bounds are known when the symmetry representation is irreducible. Multipartite collective systems, however, generally appear as reducible tensor-product representations of a symmetry algebra, which raises the question of how to determine their total uncertainty. We resolve this problem for multipartite quantum systems with a compact semisimple symmetry algebra $\mathfrak g$. Exploiting the symmetry structure, we formulate a general framework for state-independent uncertainty based on representation theory. The total variance admits an exact decomposition into intrinsic fluctuations within irreducible sectors and a nonnegative dispersion between sectors. This yields the sharp state-independent bound for the total variance $Δ_ρ^2(\mathfrak g)$ on the multipartite Hilbert space $\mathcal H$ \[ \min_ρΔ_ρ^2(\mathfrak g) = \min_{λ\inΛ(\mathcal H)} 2\langleλ,δ\rangle, \] where $ρ$ is any density operator on $\mathcal H$, $Λ(\mathcal H)$ is the set of highest weights $λ$ labeling those sectors, and $δ$ is the Weyl vector. This demonstrates that the ultimate uncertainty is completely controlled by its intrinsic symmetry structure. As a special example, this result confirms our previous conjecture that the total uncertainty floor of collective spin-$1/2$ systems depends only on the parity of the particle number. We further illustrate the framework for multipartite spin-$1$ systems, demonstrating that the same symmetry-sector mechanism persists beyond spin-$1/2$.

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