Superexchange coupling of donor qubits in silicon

TL;DR

This paper demonstrates that superexchange interactions between phosphorus donor qubits in silicon can be engineered to enable long-range, electrically tunable spin coupling, which enhances quantum computing architectures by reducing gate density and noise susceptibility.

Contribution

It introduces a method to realize and control superexchange coupling between distant donor qubits in silicon, expanding the potential for scalable quantum computing architectures.

Findings

Superexchange enables coherent spin coupling over 30-45 nm distances.

Electrical tuning of superexchange is feasible and less sensitive to charge noise.

Long-range coupling reduces gate density and noise in silicon quantum computers.

Abstract

Atomic engineering in a solid-state material has the potential to functionalize the host with novel phenomena. STM-based lithographic techniques have enabled the placement of individual phosphorus atoms at selective lattice sites of silicon with atomic precision. Here, we show that by placing four phosphorus donors spaced 10-15 nm apart from their neighbours in a linear chain, it is possible to realize coherent spin coupling between the end dopants of the chain, analogous to the superexchange interaction in magnetic materials. Since phosphorus atoms are a promising building block of a silicon quantum computer, this enables spin coupling between their bound electrons beyond nearest neighbours, allowing the qubits to be spaced out by 30-45 nm. The added flexibility in architecture brought about by this long-range coupling not only reduces gate densities but can also reduce correlated noise between qubits from local noise sources that are detrimental to error correction codes. We base our calculations on a full configuration interaction technique in the atomistic tight-binding basis, solving the 4-electron problem exactly, over a domain of a million silicon atoms. Our calculations show that superexchange can be tuned electrically through gate voltages where it is less sensitive to charge noise and donor placement errors.