The necessity of conditions for graph quantum ergodicity and Cartesian products with an infinite graph

TL;DR

This paper investigates the conditions necessary for quantum ergodicity in graphs, demonstrating that expansion and high girth alone are insufficient, and introduces new properties of Cartesian products with infinite graphs.

Contribution

It shows that expansion and high girth are not enough for quantum ergodicity and explores properties of Cartesian products involving infinite graphs.

Findings

Expansion and high girth alone do not guarantee quantum ergodicity.

New properties of Cartesian products with infinite graphs are established.

Relaxed high girth conditions are insufficient for quantum ergodicity.

Abstract

Anantharaman and Le Masson proved that any family of eigenbases of the adjacency operators of a family of graphs is quantum ergodic (a form of delocalization) assuming the graphs satisfy conditions of expansion and high girth. In this paper, we show that neither of these two conditions is sufficient by itself to necessitate quantum ergodicity. We also show that having conditions of expansion and a specific relaxation of the high girth constraint present in later papers on quantum ergodicity is not sufficient. We do so by proving new properties of the Cartesian product of two graphs where one is infinite.