Long-Range Interaction of Singlet-Triplet Qubits via Ferromagnets

TL;DR

This paper introduces a long-range interaction mechanism for singlet-triplet and spin-1/2 qubits mediated by a ferromagnet, enabling two-qubit gates at micrometer distances suitable for current semiconductor technology.

Contribution

It derives the effective Hamiltonian for qubit interactions via ferromagnets, estimates coupling strengths, and designs sequences for implementing CNOT gates with practical qubit spacing.

Findings

Operation times between 1MHz and 100MHz.

Qubit spacing of about 1 micron is feasible.

Applicable to GaAs, Silicon, and NV-center qubits.

Abstract

We propose a mechanism of a long-range coherent interaction between two singlet-triplet qubits dipolarly coupled to a dogbone-shaped ferromagnet. An effective qubit-qubit interaction Hamiltonian is derived and the coupling strength is estimated. Furthermore we derive the effective coupling between two spin-1/2 qubits that are coupled via dipolar interaction to the ferromagnet and that lie at arbitrary positions and deduce the optimal positioning. We consider hybrid systems consisting of spin-1/2 and ST qubits and derive the effective Hamiltonian for this case. We then show that operation times vary between 1MHz and 100MHz and give explicit estimates for GaAs, Silicon, and NV-center based spin qubits. Finally, we explicitly construct the required sequences to implement a CNOT gate. The resulting quantum computing architecture retains all the single qubit gates and measurement aspects of earlier approaches, but allows qubit spacing at distances of order 1$\,\mu$m for two-qubit gates, achievable with current semiconductor technology.