On the spreading of quantum walks starting from local and delocalized states

TL;DR

This paper derives analytical expressions for the long-time spreading behavior of one-dimensional quantum walks starting from various initial states, revealing how initial conditions and quantum coin phases influence dispersion.

Contribution

It provides closed-form formulas for the variance of quantum walks from different initial states and analyzes the dependence on quantum coin phases, supported by numerical simulations.

Findings

Average variance from local states is coin-independent.

Variance from Gaussian and uniform states depends on phase sums.

Quantum walks can be non-dispersive for Fourier coins.

Abstract

We investigate the ballistic spreading behavior of the one-dimensional discrete time quantum walks whose time evolution is driven by any balanced quantum coin. We obtain closed-form expressions for the long-time variance of position of quantum walks starting from any initial qubit (spin-$1/2$ particle) and position states following a delta-like (local), Gaussian and uniform probability distributions. By averaging over all spin states, we find out that the average variance of a quantum walk starting from a local state is independent of the quantum coin, while from Gaussian and uniform states it depends on the sum of relative phases between spin states given by the quantum coin, being non-dispersive for a Fourier walk and large initial dispersion. We also perform numerical simulations of the average probability distribution and variance along the time to compare them with our analytical results.