A two-qubit gate between phosphorus donor electrons in silicon*

Electron spin qubits formed by atoms in silicon have large orbital energies and weak spin-orbit coupling giving rise to isolated electron spin ground states with seconds long coherence times [1]. The exchange interaction promises fast two-qubit gate operations between single-spin qubits [2]. However, creating a tuneable exchange interaction between two electrons bound to phosphorus atom qubits has not been possible. This reflects the challenges in knowing how far apart to place the atoms to turn on and off the exchange interaction, whilst aligning atomic circuitry for high-fidelity independent readout of the spins. Here, we report a ~800 ps sqrt(SWAP) gate between phosphorus donor electron spin qubits in silicon with independent ~94 % fidelity single-shot spin readout [3]. By engineering qubit placement on the atomic scale, we provide a route to the realisation and efficient characterisation of multi-qubit quantum circuits based on donor qubits in silicon. Towards this end, we present recent results on the role of quantised nuclear spins on the exchange dynamics of atomic-scale devices.

[1] Muhonen, J. T. et al. Storing quantum information for 30 seconds in a nanoelectronic device. Nat. Nanotechnol. 9, 986--991 (2014)
[2] Loss, D. & DiVincenzo, D. P. Quantum computation with quantum dots. Phys. Rev. A 57, 120--126 (1998)
[3] Y He et al. A two-qubit gate between phosphorus donor electrons in silicon. Nature 571, 371--375 (2019)