Experimental simulation and limitations of quantum walks with trapped ions

TL;DR

This paper analyzes the limitations of discrete quantum walks with trapped ions, especially deviations from ideal behavior due to higher-order effects, and proposes a new laser pulse scheme to overcome these constraints for scalable quantum walk experiments.

Contribution

It identifies the limitations of existing quantum walk protocols with trapped ions and introduces a new scheme using short laser pulses to enable larger and higher-dimensional quantum walks.

Findings

Higher-order effects dominate after a few steps, limiting scalability.

Experimental results confirm deviations from ideal quantum walks.

The proposed scheme can increase the number of steps to over 100.

Abstract

We examine the prospects of discrete quantum walks (QWs) with trapped ions. In particular, we analyze in detail the limitations of the protocol of Travaglione and Milburn (PRA 2002) that has been implemented by several experimental groups in recent years. Based on the first realization in our group (PRL 2009), we investigate the consequences of leaving the scope of the approximations originally made, such as the Lamb--Dicke approximation. We explain the consequential deviations from the idealized QW for different experimental realizations and an increasing number of steps by taking into account higher-order terms of the quantum evolution. It turns out that these become dominant after a few steps already, which is confirmed by experimental results and is currently limiting the scalability of this approach. Finally, we propose a new scheme using short laser pulses, derived from a protocol from the field of quantum computation. We show that the new scheme is not subject to the above-mentioned restrictions, and analytically and numerically evaluate its limitations, based on a realistic implementation with our specific setup. Implementing the protocol with state-of-the-art techniques should allow for substantially increasing the number of steps to 100 and beyond and should be extendable to higher-dimensional QWs.