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. The exchange interaction promises fast two-qubit gate operations between single-spin qubits. Until now, creating a tunable 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 read out of the spins. Here we report a ~800 ps √SWAP gate between phosphorus donor electron spin qubits in silicon with independent ~94 % fidelity single shot spin read-out. 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.