Majorana sweet spots in 3-site Kitaev chains

TL;DR

This paper explores 3-site Kitaev chains and identifies three types of sweet spots that optimize Majorana bound state properties, enhancing stability and control for quantum computing applications.

Contribution

It introduces a theoretical analysis of 3-site Kitaev chains revealing three distinct sweet spots with different MBS localization and robustness characteristics.

Findings

Identified three types of sweet spots in 3-site Kitaev chains.

Genuine 3-site sweet spots offer the highest stability.

Distinct spectral and transport signatures differentiate the sweet spot types.

Abstract

Minimal Kitaev chains, composed of two quantum dots (QDs) connected via a superconductor, have emerged as an attractive platform to realize Majorana bound states (MBSs). These excitations exist when the ground state is degenerate. The additional requirement of isolating the MBS wavefunctions further restricts the parameter space to discrete sweet spots. While scaling up to Kitaev chains with more than two sites has the potential to improve the stability of the MBSs, longer chains offer more features to optimize, including the MBS localization length and the excitation gap. In this work, we theoretically investigate 3-site Kitaev chains and show that there are three different types of sweet spots, obtained by maximizing distinct MBS properties: genuine 3-site sweet spots with well-localized MBSs at the ends, effective 2-site sweet spots, where the middle site acts as a barrier, and sweet spots with delocalized MBSs that overlap in the middle of the chain. These three cases feature different degrees of robustness against perturbations, with the genuine 3-site being the most stable. We analyze the energy spectrum, transport, and microwave absorption associated with these three cases, showing how to distinguish them.