Localization measures of parity adapted U($D$)-spin coherent states applied to the phase space analysis of the $D$-level Lipkin-Meshkov-Glick model

TL;DR

This paper investigates phase-space localization measures of parity-adapted U(D)-spin coherent states to analyze quantum phase transitions in D-level Lipkin-Meshkov-Glick models, emphasizing the role of Husimi functions and entropy.

Contribution

It introduces a phase-space approach using U(D)-spin coherent states and Husimi functions to study quantum phase transitions in D-level systems, extending previous methods to higher-dimensional models.

Findings

Husimi functions effectively visualize quantum phase transitions.

Parity projection yields Schrödinger cat states that approximate eigenstates.

Localization measures serve as markers for critical points.

Abstract

We study phase-space properties of critical, parity symmetric, $N$-quDit systems undergoing a quantum phase transition (QPT) in the thermodynamic $N\to\infty$ limit. The $D=3$ level (qutrit) Lipkin-Meshkov-Glick (LMG) model is eventually examined as a particular example. For this purpose, we consider U$(D)$-spin coherent states (DSCS), generalizing the standard $D=2$ atomic coherent states, to define the coherent state representation $Q_\psi$ (Husimi function) of a symmetric $N$-quDit state $|\psi>$ in the phase space $\mathbb CP^{D-1}$ (complex projective manifold). DSCS are good variational aproximations to the ground state of a $N$-quDit system, specially in the $N\to\infty$ limit, where the discrete parity symmetry $\mathbb{Z}_2^{D-1}$ is spontaneously broken. For finite $N$, parity can be restored by projecting DSCS onto $2^{D-1}$ different parity invariant subspaces, which define generalized ``Schr\"odinger cat states'' reproducing quite faithfully low-lying Hamiltonian eigenstates obtained by numerical diagonalization. Precursors of the QPT are then visualized for finite $N$ by plotting the Husimi function of these parity projected DSCS in phase space, together with their Husimi moments and Wehrl entropy, in the neighborhood of the critical points. These are good localization measures and markers of the QPT.