Strain engineering of Andreev spin qubits in Germanium

TL;DR

This paper demonstrates that strain engineering in germanium heterostructures can significantly enhance spin splitting, enabling the realization of Andreev spin qubits with GHz-range splittings and rapid all-electric quantum gates.

Contribution

It introduces strain engineering as a novel approach to improve spin splitting in germanium-based Josephson junctions for quantum computing.

Findings

Strain suppresses spin splitting in current germanium devices.

Unstrained and tensile-strained structures significantly increase spin-orbit effects.

Predicted GHz-range spin splittings enable fast quantum gates.

Abstract

Planar germanium heterostructures are promising hosts for hybrid quantum devices due to their compatibility with superconductors, low material disorder, and relaxed fabrication constraints. Also, the potentially low density of nuclear spins and strong spin-orbit interaction make germanium attractive for coherent spin physics. However, recent microwave spectroscopy experiments were unable to resolve a spin-splitting of bound states in germanium Josephson junctions, the prerequisite for defining and controlling Andreev spin qubits. Here, we argue that compressive strain is the key mechanism suppressing spin splitting in current devices. Furthermore, we propose unstrained and tensile-strained heterostructures, fully compatible with state-of-the-art growth technology, that significantly enhance the relevant spin-orbit effect. By numerically simulating ballistic Josephson junctions, we predict spin splittings comfortably in the GHz range, more than 2 orders of magnitude larger than compressively strained cases, and all-electric quantum gates in a hundred nanoseconds. Our results establish strain engineering as a key design principle for realizing Andreev spin qubits in germanium-based devices.