Scalable quantum processors require high-fidelity universal quantum logic operations in a manufacturable physical platform. Donors in silicon provide atomic size, excellent quantum coherence and compatibility with standard semiconductor processing, but no entanglement between donor-bound electron spins has been demonstrated to date. Here we present the experimental demonstration and tomography of universal 1- and 2-qubit gates in a system of two weakly exchange-coupled electrons, bound to single phosphorus donors introduced in silicon by ion implantation. We surprisingly observe that the exchange interaction has no effect on the qubit coherence. We quantify the fidelity of the quantum operations using gate set tomography (GST), and we use the universal gate set to create entangled Bell states of the electrons spins, with fidelity ~ 93%, and concurrence 0.91 +/- 0.08. These results form the necessary basis for scaling up donor-based quantum computers.
@article{arxiv.2309.15463,
title = {Tomography of entangling two-qubit logic operations in exchange-coupled donor electron spin qubits},
author = {Holly G. Stemp and Serwan Asaad and Mark R. van Blankenstein and Arjen Vaartjes and Mark A. I. Johnson and Mateusz T. Mądzik and Amber J. A. Heskes and Hannes R. Firgau and Rocky Y. Su and Chih Hwan Yang and Arne Laucht and Corey I. Ostrove and Kenneth M. Rudinger and Kevin Young and Robin Blume-Kohout and Fay E. Hudson and Andrew S. Dzurak and Kohei M. Itoh and Alexander M. Jakob and Brett C. Johnson and David N. Jamieson and Andrea Morello},
journal= {arXiv preprint arXiv:2309.15463},
year = {2025}
}