Genetic Design of Enhanced Valley Splitting towards a Spin Qubit in Silicon

TL;DR

This paper demonstrates that ordered Ge-Si layered barriers can significantly enhance valley splitting in silicon quantum wells, enabling more reliable spin qubits for quantum computing through a genetic algorithm optimized design.

Contribution

It introduces a novel atomistic design of superlattice barriers using genetic algorithms to maximize valley splitting in silicon quantum wells.

Findings

Valley splitting can be increased up to ~9 meV with optimized layer sequences.

Ordered Ge-Si superlattices outperform random alloy barriers in valley splitting.

Enhanced valley splitting is robust against inter-layer mixing.

Abstract

Electronic spins in Silicon (Si) are rising contenders for qubits -- the logical unit of quantum computation-- owing to its outstanding spin coherence properties and compatibility to standard electronics. A remarkable limitation for spin quantum computing in Si hosts is the orbital degeneracy of this material's conduction band, preventing the spin-1/2 states from being an isolated two-level system. So far available samples of Si quantum wells cladded by Ge-Si alloy barriers provide relatively small valley splitting (VS), with the order of 1 meV or less, degrading the fidelity of qubits encoded in spin "up" and "down" states in Si. Here, based on an atomically resolved pseudopotential theory, we demonstrate that ordered Ge-Si layered barriers confining a Si slab can be harnessed to enhance the VS in the active Si region by up to one order of magnitude compared to the random alloy barriers adopted so far. A biologically inspired genetic-algorithm search is employed to identify magic Ge/Si layer sequences of the superlattice barriers that isolate the electron ground state in a single valley composition with VS as large as ~9 meV. The enhanced VS is preserved with the reasonable inter-layer mixing between different species, and is interestingly "protected" even if some larger mixing occurs. Implementation of the optimized layer sequences of barriers, within reach of modern superlattice growth techniques, overcomes in a practical systematic way the main current limitations related to the orbital degeneracy, thus providing a roadmap for reliable spin-only quantum computing in Si.