Nature Nanotech. http://doi.org/9pq (2015)

Bell states feature the maximally achievable degree of entanglement, a necessary resource for the implementation of efficient quantum computation algorithms. In this kind of system, the state of two distinct particles cannot be described as a product of the states of its local constituents. John Bell in 1964 defined the upper limit for such correlations to occur assuming classical properties — locality and realism; violation of Bell's theorem is therefore directly linked to quantum entanglement. Andrea Morello and co-workers now demonstrate entanglement between two-level systems (qubits) in a silicon platform. The researchers prepared a maximally entangled state using the electron and the nuclear spins of a single phosphorus atom as qubits; the two-spin initialization fidelity was shown to reach 97%. Two-qubit state tomography used to map out the density matrix of the entangled states revealed very high, near-unity fidelity of 96% when compared to the ideal states. This demonstration of high-level manipulation of silicon-based qubits constitutes an important step towards real-life quantum computing.