  Quantum entanglement between spatially separated objects is one of the most
intriguing phenomena in physics. The outcomes of independent measurements on
entangled objects show correlations that cannot be explained by classical
physics. Besides being of fundamental interest, entanglement is a unique
resource for quantum information processing and communication. Entangled qubits
can be used to establish private information or implement quantum logical
gates. Such capabilities are particularly useful when the entangled qubits are
spatially separated, opening the opportunity to create highly connected quantum
networks or extend quantum cryptography to long distances. Here we present a
key experiment towards the realization of long-distance quantum networks with
solid-state quantum registers. We have entangled two electron spin qubits in
diamond that are separated by a three-meter distance. We establish this
entanglement using a robust protocol based on local creation of spin-photon
entanglement and a subsequent joint measurement of the photons. Detection of
the photons heralds the projection of the spin qubits onto an entangled state.
We verify the resulting non-local quantum correlations by performing
single-shot readout on the qubits in different bases. The long-distance
entanglement reported here can be combined with recently achieved
initialization, readout and entanglement operations on local long-lived nuclear
spin registers, enabling deterministic long-distance teleportation, quantum
repeaters and extended quantum networks.
