Spin interactions and switching in vertically tunnel-coupled quantum dots

TL;DR

This paper analyzes how magnetic and electric fields influence spin exchange coupling in vertically tunnel-coupled quantum dots, revealing conditions for singlet-triplet transitions and methods for spin control crucial for quantum computing.

Contribution

It provides a theoretical framework for understanding and controlling spin interactions in quantum dots, including a novel electric field switching method for quantum information applications.

Findings

Strong decrease of exchange coupling J with increasing in-plane magnetic field

Observation of singlet-triplet crossing as a jump in magnetization

Method to switch spin coupling on and off using electric fields in coupled dots

Abstract

We determine the spin exchange coupling J between two electrons located in two vertically tunnel-coupled quantum dots, and its variation when magnetic (B) and electric (E) fields (both in-plane and perpendicular) are applied. We predict a strong decrease of J as the in-plane B field is increased, mainly due to orbital compression. Combined with the Zeeman splitting, this leads to a singlet-triplet crossing, which can be observed as a pronounced jump in the magnetization at in-plane fields of a few Tesla, and perpendicular fields of the order of 10 Tesla for typical self-assembled dots. We use harmonic potentials to model the confining of electrons, and calculate the exchange J using the Heitler-London and Hund-Mulliken technique, including the long-range Coulomb interaction. With our results we provide experimental criteria for the distinction of singlet and triplet states and therefore for microscopic spin measurements. In the case where dots of different sizes are coupled, we present a simple method to switch on and off the spin coupling with exponential sensitivity using an in-plane electric field. Switching the spin coupling is essential for quantum computation using electronic spins as qubits.