Scaling and crossover behaviour in a truncated long range quantum walk

Abstract

We consider a discrete time quantum walker in one dimension, where at each step, the step length $\ell$ is chosen from a distribution $P(\ell) \propto \ell^{-\delta -1}$ with $\ell \leq \ell_{max}$. We evaluate the probability $f(x,t)$ that the walker is at position $x$ at time $t$ and its first two moments. As expected, the disorder effectively localizes the walk even for large values of $\delta$. Asymptotically, $\langle x^2 \rangle \propto t^{3/2}$ and $\langle x \rangle \propto t^{1/2}$ independent of $\delta$ and $\ell$, both finite. The scaled distribution $f(x,t)t^{1/2}$ plotted versus $x/t^{1/2}$ shows a data collapse for $x/t < \alpha(\delta,\ell_{max}) \sim \mathcal O(1) $ indicating the existence of a universal scaling function. The scaling function is shown to have a crossover behaviour at $\delta = \delta^* \approx 4.0$ beyond which the results are independent of $\ell_{max}$. We also calculate the von Neumann entropy of entanglement which gives a larger asymptotic value compared to the quantum walk with unique step length even for large $\delta$, with negligible dependence on the initial condition.