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

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.