Controlling Hole Spin in Quantum Dot Molecules: Random Alloy GaBiAs Inter-dot Barriers

Hole spins in InAs quantum dots (QDs) allow all-optical control and have long lifetimes, qualifying them as excellent qubit candidates. Furthermore, quantum dot molecules (QDMs) created by vertically stacking two QDs introduce an inter-dot barrier, which can be used to mediate spin-mixing between respective dot states. As spin-mixing is essential for well-designed qubits, we seek to further enhance hole spin-mixing in QDM systems by introducing a low concentration of Bi in the inter-dot region. Barriers containing GaBiAs, compared to GaAs, have a higher hole tunneling rate while minimally affecting electrons or split off bands. Using an atomistic tight-binding model, we show that a three-fold increase in hole tunnel coupling is achieved for well-designed barriers. We also show that this leads to a significant enhancement in hole spin-mixing, and allows for control schemes that are not possible with pure GaAs barriers. Finally, we present a breakdown of various compounding effects, such as strain, Bi electronic structure, or spin-orbit coupling, and how they contribute to the final hole tunnel-coupling and spin-mixing.