Scalable entanglement of nuclear spins mediated by electron exchange

The use of nuclear spins for quantum computation is limited by the
difficulty in creating genuine quantum entanglement between distant
nuclei. Current demonstrations of nuclear entanglement in semiconductors
rely upon coupling the nuclei to a common electron, which is not a
scalable strategy. Here we demonstrate a two-qubit Control-Z logic
operation between the nuclei of two phosphorus atoms in a silicon device,
separated by up to 20 nanometers. Each atom binds separate electrons,
whose exchange interaction mediates the nuclear two-qubit gate. We prove
that the nuclei are entangled by preparing and measuring Bell states with
a fidelity of 76 +/- 5% and a concurrence of 0.67+/- 0.05. With this
method, future progress in scaling up semiconductor spin qubits can be
extended to the development of nuclear-spin based quantum computers.