Tailoring Germanium Heterostructures for Quantum Devices with Machine Learning

TL;DR

This paper demonstrates how machine learning-driven optimization of Ge heterostructures can significantly enhance spin-orbit interaction and qubit performance for quantum devices.

Contribution

It introduces a method to modify Ge heterostructures with silicon spikes, optimized via Bayesian methods, to improve spin-orbit interaction and quantum device performance.

Findings

Enhanced spin-orbit interaction by up to three orders of magnitude.

Achieved two orders of magnitude higher qubit quality factors.

Predicted GHz-scale spin splittings for hybrid qubits.

Abstract

Germanium (Ge) quantum wells are emerging as versatile platforms for quantum devices, supporting high-quality spin qubits and integration with superconducting leads. These applications benefit from strong intrinsic spin-orbit interaction (SOI), enabling efficient electrical control and engineering of spin degrees of freedom. The most advanced Ge/SiGe heterostructures to date, based on compressively strained Ge channels within strain-relaxed silicon-germanium (SiGe) barriers, exhibit weak SOI due to the heavy-hole character of the wave function, posing challenges for spin-based quantum devices and requiring complex device designs for fast qubit manipulation. In this work, we demonstrate that concrete heterostructure modifications can overcome these limitations, enhancing SOI by up to three orders of magnitude. Specifically, we propose to enrich unstrained Ge channels by localized, strained silicon spikes. Leveraging a multi-objective Bayesian optimization, we optimize the spike profile to maximize SOI, while ensuring compatibility with current epitaxial growth processes and robustness against realistic variations of growth parameters. Our heterostructure substantially enhances device performance, yielding up to two orders of magnitude higher quantum-dot spin qubit quality factors than state-of-the-art materials. We also predict GHz-scale spin splittings for hybrid superconducting Andreev spin qubits. These novel Ge heterostructures with engineered Si concentration profiles can open pathways to scalable quantum and spintronic applications.