Identification of quantum scars via phase-space localization measures

Quantum 6, 644 (2022).

https://doi.org/10.22331/q-2022-02-08-644

There is no unique way to quantify the degree of delocalization of quantum states in unbounded continuous spaces. In this work, we explore a recently introduced localization measure that quantifies the portion of the classical phase space occupied by a quantum state. The measure is based on the $alpha$-moments of the Husimi function and is known as the Rényi occupation of order $alpha$. With this quantity and random pure states, we find a general expression to identify states that are maximally delocalized in phase space. Using this expression and the Dicke model, which is an interacting spin-boson model with an unbounded four-dimensional phase space, we show that the Rényi occupations with $alpha>1$ are highly effective at revealing quantum scars. Furthermore, by analyzing the high moments ($alpha>1$) of the Husimi function, we are able to identify qualitatively and quantitatively the unstable periodic orbits that scar some of the eigenstates of the model.