Index Theorems for One-dimensional Chirally Symmetric Quantum Walks with Asymptotically Periodic Parameters

TL;DR

This paper develops an index theory for one-dimensional chirally symmetric quantum walks with asymptotically periodic parameters, extending previous models by replacing the 2-phase condition with a more general asymptotic periodicity assumption.

Contribution

It introduces a new framework for bulk-edge correspondence in quantum walks using asymptotically periodic parameters instead of the traditional 2-phase condition.

Findings

Established a topological invariant analysis for Toeplitz operators.

Revised the bulk-edge correspondence under the asymptotically periodic assumption.

Provided lower bounds for symmetry-protected edge states.

Abstract

We focus on index theory for chirally symmetric discrete-time quantum walks on the one-dimensional integer lattice. Such a discrete-time quantum walk model can be characterised as a pair of a unitary self-adjoint operator $\varGamma$ and a unitary time-evolution operator $U,$ satisfying the chiral symmetry condition $U^* = \varGamma U \varGamma.$ The significance of this index theory lies in the fact that the index we assign to the pair $(\varGamma,U)$ gives a lower bound for the number of symmetry protected edge-states associated with the time-evolution $U.$ The symmetry protection of edge-states is one of the important features of the bulk-edge correspondence. The purpose of the present paper is to revisit the well-known bulk-edge correspondence for the split-step quantum walk on the one-dimensional integer lattice. The existing mathematics literature makes use of a fundamental assumption, known as the $2$-phase condition, but we completely replace it by the so-called asymptotically periodic assumption in this article. This generalisation heavily relies on analysis of some topological invariants associated with Toeplitz operators.