Entangling electrons and nuclei in a four-qubit, two-atom device in silicon*

Ion-implanted 31P donors in silicon have demonstrated exceptionally long coherence times, and >99% gate fidelities, including >99% 2-qubit fidelity for nuclear spins coupled through a common electron at ~6 nm distance. Extending the spatial range of 2-qubit gates requires using the electron exchange interaction. Here we present a four-qubit system (two electrons, two 31P nuclei) consisting of two exchange-coupled phosphorus donor atoms in silicon, interacting over a distance of 18.5+-3.5 nm. We create and benchmark Bell states to show that this coupling can be used to entangle both the electrons and nuclei. The electron Bell state fidelity is 93 +- 9% [1]. Nuclear spins are entangled via a controlled-Z gate, created through conditional electron spin rotations. The nuclear Bell state fidelity seems severely limited by SPAM errors. Future efforts will incorporate the Maxwell’s demon initialization method [2] to reduce SPAM and achieve entanglement of all four qubits.

[1] H. Stemp et al. arXiv:2309.15463 (2023)

[2] M. Johnson et al., Phys. Rev. X 12, 041008 (2022)