Almost reducibility and oscillatory growth of Sobolev norms

TL;DR

This paper investigates the almost reducibility of a 1D quantum harmonic oscillator with quasi-periodic perturbations, establishing bounds on Sobolev norm growth and demonstrating the optimality of these bounds through oscillatory behavior.

Contribution

It introduces a novel analysis of Sobolev norm growth for perturbed quantum harmonic oscillators, including optimal bounds and oscillatory growth examples.

Findings

Established an $o(t^s)$ upper bound for Sobolev norm growth.

Proved the existence of perturbations with oscillatory growth close to $t^s$.

Demonstrated the optimality of the growth bounds.

Abstract

For 1D quantum harmonic oscillator perturbed by a time quasi-periodic quadratic form of $(x,-{\rm i}\partial_x)$, we show its almost reducibility. The growth of Sobolev norms of solution is described based on the scheme of almost reducibility. In particular, an $o(t^s)-$upper bound is shown for the $\CH^s-$norm if the equation is non-reducible. Moreover, by Anosov-Katok construction, we also show the optimality of this upper bound, i.e., the existence of quasi-periodic quadratic perturbation for which the growth of ${\mathcal H}^s-$norm of the solution is $o(t^s)$ as $t\to\infty$ but arbitrarily ``close" to $t^s$ in an oscillatory way.