# Line-Graph Lattices: Euclidean and Non-Euclidean Flat Bands, and Implementations in Circuit Quantum Electrodynamics

## TL;DR

This paper explores line-graph lattices in Euclidean and hyperbolic spaces, revealing flat bands and localized states, and demonstrates their realization in circuit quantum electrodynamics with superconducting resonators.

## Contribution

It combines mathematical and physical approaches to analyze line-graph lattices, introduces criteria for gap maximization, and presents a hardware implementation in circuit QED.

## Key findings

- Identification of flat bands and localized eigenstates in line-graph lattices.
- Development of classification criteria for lattice phenomenology.
- Experimental realization using superconducting coplanar waveguide resonators.

## Abstract

Materials science and the study of the electronic properties of solids are a major field of interest in both physics and engineering. The starting point for all such calculations is single-electron, or non-interacting, band structure calculations, and in the limit of strong on-site confinement this can be reduced to graph-like tight-binding models. In this context, both mathematicians and physicists have developed largely independent methods for solving these models. In this paper we will combine and present results from both fields. In particular, we will discuss a class of lattices which can be realized as line graphs of other lattices, both in Euclidean and hyperbolic space. These lattices display highly unusual features including flat bands and localized eigenstates of compact support. We will use the methods of both fields to show how these properties arise and systems for classifying the phenomenology of these lattices, as well as criteria for maximizing the gaps. Furthermore, we will present a particular hardware implementation using superconducting coplanar waveguide resonators that can realize a wide variety of these lattices in both non-interacting and interacting form.