Subgap states in semiconductor-superconductor devices for quantum technologies: Andreev qubits and minimal Majorana chains

TL;DR

Recent experimental progress in semiconductor-superconductor devices has enabled precise control over subgap states, facilitating the development of novel qubits and advancing the pursuit of Majorana-based quantum computing.

Contribution

This paper reviews recent advances in engineering subgap states, including experimental demonstrations of Andreev qubits and minimal Majorana chains, highlighting new quantum device designs.

Findings

Demonstration of Andreev qubits using quantum dot Josephson junctions

Realization of minimal Kitaev chains with coupled quantum dots

Subgap states can be exploited for quantum coherence and qubit development

Abstract

In recent years, experimental advances have made it possible to achieve an unprecedented degree of control over the properties of subgap bound states in hybrid nanoscale superconducting structures. This research has been driven by the promise of engineering subgap states for quantum applications, which includes Majorana zero modes predicted to appear at the interface of superconductor and other materials, like topological insulators or semiconductors. In this chapter, we revise the status of the field towards the engineering of quantum devices in controllable semiconductor-superconductor heterostructures. We begin the chapter with a brief introduction about subgap states, focusing on their mathematical formulation. After introducing topological superconductivity using the Kitaev model, we discuss the advances in the search for Majorana states over the last few years, highlighting the difficulties of unambiguously distinguish these states from nontopological subgap states. In recent years, the precise engineering of bound states by a bottom-up approach using quantum dots has led to unprecedented experimental advances, including experimental demonstrations of an Andreev qubits based on a quantum dot Josephson junction and a minimal Kitaev chain based on two quantum dots coherently coupled by the bound states of an intermediate superconducting segment. These experimental advances have revitalized the field and helped to understand that, far from being a disadvantage, the presence of subgap bound states can be exploited for new qubit designs and quantum coherence experiments, including Majorana-based qubits.