Spreading Estimates for Quantum Walks on the Integer Lattice via Power-Law Bounds on Transfer Matrices

TL;DR

This paper develops new mathematical bounds for quantum walk spreading rates on the integer lattice, applying transfer matrix estimates to demonstrate anomalous and ballistic transport in specific quantum walk models.

Contribution

It introduces transfer matrix-based spreading bounds for quantum walks and applies them to Fibonacci and periodic coin distributions, revealing anomalous and ballistic transport behaviors.

Findings

Fibonacci coin distribution leads to anomalous transport.

Periodic coin distribution results in ballistic transport.

First concrete example of anomalous quantum walk transport.

Abstract

We discuss spreading estimates for dynamical systems given by the iteration of an extended CMV matrix. Using a connection due to Cantero--Gr\"unbaum--Moral--Vel\'azquez, this enables us to study spreading rates for quantum walks in one spatial dimension. We prove several general results which establish quantitative upper and lower bounds on the spreading of a quantum walk in terms of estimates on a pair of associated matrix cocycles. To demonstrate the power and utility of these methods, we apply them to several concrete cases of interest. In the case where the coins are distributed according to an element of the Fibonacci subshift, we are able to rather completely describe the dynamics in a particular asymptotic regime. As a pleasant consequence, this supplies the first concrete example of a quantum walk with anomalous transport, to the best of our knowldege. We also prove ballistic transport for a quantum walk whose coins are periodically distributed.