Tomography of entangling two-qubit logic operations in exchange-coupled donor electron spin qubits

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.