Building topological quantum circuits: Majorana nanowire junctions

TL;DR

This paper investigates the effects of wire-junctions in topological quantum computing with Majorana zero modes, identifying issues with spurious states and proposing 2D electron gas structures for more robust topological properties.

Contribution

It reveals how T junctions can introduce non-topological subgap states and proposes 2D electron gas-based junctions to achieve stable topological features without spurious zero modes.

Findings

T junctions can create non-topological subgap states

2D electron gas junctions can host robust topological states

Proposed structures enable experiments to clarify zero-bias peaks

Abstract

Topological quantum computation using non-Abelian Majorana zero modes localized in proximitized semiconductor nanowires requires careful electrostatic control of wire-junctions so as to manipulate and braid the zero modes enabling anyonic fault-tolerant gate operations. We theoretically investigate the topological superconducting properties of such elementary wire-junctions, the so-called T junctions, finding that the existence of the junction may nonperturbatively affect the Majorana behavior by introducing spurious non-topological subgap states mimicking zero-modes. We propose a possible solution to this potentially serious problem by showing that junctions made lithographically from two-dimensional (2D) electron gas systems may manifest robust subgap topological properties without any spurious zero modes. We propose a 2D structure that enables multiprobe tunneling experiments providing position-dependent spectroscopy, which can decisively settle outstanding open questions related to the origin of the zero-bias conductance peaks observed experimentally. We also find that junctions with trivial superconductors may result in local perturbations that induce extrinsic low-energy states similar to those associated with wire junctions.