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arXiv:2609.18919

Wehrl-type entropy problem for compact connected semisimple Lie groups

Haonan Zhang

Published Sep 16, 2026• 0 citations
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Abstract

Let $G$ be a compact connected semisimple Lie group, and let $π:G\to U(V_λ)$ be a finite-dimensional irreducible unitary representation associated with the highest weight $λ$. We prove that coherent projectors are the unique minimizers of the Wehrl entropy over all density matrices on $V_λ$. Similar extremal properties hold for power functions in place of the entropy function. The proof uses a second variation at the extremizer by perturbation in directions associated with Killing fields, similar to the strategy of Frank and Lieb used in \cite{FrankLieb}. Then the problem reduces to the uncertainty principle characterizing coherent states. This solves the Wehrl-type entropy problem for arbitrary compact connected semisimple Lie groups.

DOI
10.48550/arxiv.2609.18919

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Let GGG be a compact connected semisimple Lie group, and let π:G→U(Vλ)π:G\to U(V_λ)π:G→U(Vλ​) be a finite-dimensional irreducible unitary representation associated with the highest weight λλλ. We prove that coherent projectors are the unique minimizers of the Wehrl entropy over all density matrices on VλV_λVλ​. Similar extremal properties hold for power functions in place of the entropy function. The proof uses a second variation at the extremizer by perturbation in directions associated with Killing fields, similar to the strategy of Frank and Lieb used in \cite{FrankLieb}. Then the problem reduces to the uncertainty principle characterizing coherent states. This solves the Wehrl-type entropy problem for arbitrary compact connected semisimple Lie groups.
Wehrl used Glauber coherent states to define a map from quantum density matrices to classical phase space densities and conjectured that for Glauber coherent states the mininimum classical entropy would occur for density matrices equal to projectors onto coherent states. This was proved by Lieb in 1978 who also extended the conjecture to Bloch SU(2) spin-coherent states for every angular momentum J. This conjecture is proved here. We also recall our 1991 extension of the Wehrl map to a quantum channel from J to K=J+12,J+1,..., with K=∞ corresponding to the Wehrl map to classical densities. These channels were later recognized as the optimal quantum cloning channels. For each J and J<K⩽∞ we show that the minimal output entropy for the channels occurs for a J coherent state. We also show that coherent states both Glauber and Bloch minimize any concave functional, not just entropy.
A construction of the Wehrl entropy is proposed for an arbitrary locally compact abelian group GGG . It is proved that the Wehrl entropy is not less than a certain nonnegative integer, which is an invariant of the group GGG . The minimum of the Wehrl entropy is attained on coherent states.
We consider the problem of the stability (with sharp exponent) of the Lieb--Solovej inequality for symmetric SU(N)SU(N)SU(N) coherent states, which was obtained only recently by the authors. Here, we propose an elementary proof of this result, based on reformulating the Wehrl-type entropy as a function defined on the unit sphere in Cd\mathbb{C}^dCd, for some suitable ddd, and on some explicit (and somewhat surprising) computations.
A Wehrl entropy construction is proposed for an arbitrary locally compact abelian group GGG. It is proved that the Wehrl entropy is not less than a non-negative integer, which is an invariant of the group GGG. The minimum of the Wehrl entropy is achieved on coherent states.
We focus on quantum systems represented by a Hilbert space L 2 ( A ) , where A is a locally compact Abelian group that contains a compact open subgroup. We examine two interconnected issues related to Weyl-Heisenberg operators. First, we provide a complete and elegant solution to the problem of describing the stabilizer states in terms of their wave functions — an issue that arises in quantum information theory. Subsequently, we demonstrate that the stabilizer states are exactly the minimizers of the Wehrl entropy, thereby solving the Wehrl-type entropy conjecture for any such group (in particular, for finite-dimensional vector spaces over non-Archimedean local fields). Additionally, we construct a moduli space for the set of stabilizer states, that is, a parametrization of this set, that endows it with a natural algebraic structure, and we derive a formula for the number of stabilizer states when A is finite. Indeed, these results are novel even for finite Abelian groups.