# Light-cone and local front dynamics of a single-particle extended quantum walk

## TL;DR

This paper analyzes the light-cone and front dynamics of a single-particle extended quantum walk on a 1D lattice, revealing anomalous sub-diffusive scaling, phase transitions in causal cones, and connections to spin chain physics.

## Contribution

It provides a detailed characterization of wave front behavior, including phase transitions and internal staircase structures, in extended quantum walks with finite range hopping.

## Key findings

- Identification of ordinary and extremal fronts with anomalous scaling
- Existence of a causal light-cone with sub-diffusive front spreading
- Transition from one to two causal cones at a critical hopping strength

## Abstract

We study the light-cone and front dynamics of a single particle continuous time extended quantum walk on a one dimensional lattice with finite range hopping. We show that, in general, for an initially localized state, propagating wave fronts can be characterized as ordinary or extremal fronts with the latter exhibiting an anomalous sub-diffusive scaling behaviour in the front region. We investigate the dynamical global and local scaling properties of the cumulative probability distribution function for the extended walk with nearest and next-nearest neighbour hopping using analytical and numerical methods. The global scaling shows the existence of a 'causal light-cone' corresponding to excitations travelling with a velocity smaller than a maximal 'light velocity'. Maximal fronts moving with fixed 'light velocity' bound the causal cone. The front regions spread with time sub-diffusively exhibiting a local Airy scaling which leads to an internal staircase structure. At a certain critical next-nearest neighbour hopping strength, there is a transition from a phase with one 'causal cone' to a phase with two nested 'causal cones' and the existence of an internal staircase structure in the corresponding cumulative distribution profiles. We also connect the study to that in spin chain systems and indicate that a single particle quantum walk on the one dimensional lattice already captures the many body physics of a spin chain system. In particular, we suggest that the time evolution of a single particle quantum walk on the one dimensional lattice with an initially localized state is equivalent to the time evolution of a domain wall initial state in a corresponding spin chain system.