# A Brief Introduction to Stationary Quantum Chaos in Generic Systems

Published: **28. July 2020**

Category: **Natural Sciences**

Author: **Marko Robnik**

**ABSTRACT**

We review the basic aspects of quantum chaos (wave chaos) in mixed-type Hamiltonian

systems with divided phase space, where regular regions containing the invariant tori coexist

with the chaotic regions. The quantum evolution of classically chaotic bound systems does

not possess the sensitive dependence on initial conditions, and thus no chaotic behaviour

occurs, as the motion is always almost periodic. However, the study of the stationary

solutions of the Schr¨odinger equation in the quantum phase space (Wigner functions or

Husimi functions) reveals precise analogy of the structure of the classical phase portrait. In

classically integrable regions the spectral (energy) statistics is Poissonian, while in the ergodic chaotic regions the random matrix theory applies. If we have the mixed-type classical phase space, in the semiclassical limit (short wavelength approximation) the spectrum is composed of Poissonian level sequence supported by the regular part of the phase space, and chaotic sequences supported by classically chaotic regions, being statistically independent of each other, as described by the Berry–Robnik distribution. In quantum systems with discrete energy spectrum the Heisenberg time tH = 2π~/∆E, where ∆E is the mean level spacing (inverse energy level density), is an important time scale. The classical transport time scale tT (transport time) in relation to the Heisenberg time scale tH (their ratio is the parameter α = tH/tT ) determines the degree of localization of the chaotic eigenstates, whose measure A is based on the information entropy. We show that A is linearly related to the normalized inverse participation ratio. We study the structure of quantum localized chaotic eigenstates (their Wigner and Husimi functions) and the distribution of localization measure A. The latter one is well described by the beta distribution, if there are no sticky regions in the classical phase space. Otherwise, they have a complex nonuniversal structure. We show that the localized chaotic states display the fractional power-law repulsion between the nearest energy levels in the sense that the probability density (level spacing distribution) to find successive levels on a distance S goes like ∝ S β for small S, where 0 ≤ β ≤ 1, and β = 1 corresponds to completely extended states, and β = 0 to the maximally localized states. β goes from 0 to 1 when α goes from 0 to ∞. β is a function of hAi, as demonstrated in the quantum kicked rotator, the stadium billiard, and a mixed-type billiard.

Keywords: quantum chaos, Hamiltonian system, random matrix theory, mixed-type classical phase space, spectral statistics, quantum localization

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