  The realization of a network of quantum registers is an outstanding challenge
in quantum science and technology. We experimentally investigate a network node
that consists of a single nitrogen-vacancy (NV) center electronic spin
hyperfine-coupled to nearby nuclear spins. We demonstrate individual control
and readout of five nuclear spin qubits within one node. We then characterize
the storage of quantum superpositions in individual nuclear spins under
repeated application of a probabilistic optical inter-node entangling protocol.
We find that the storage fidelity is limited by dephasing during the electronic
spin reset after failed attempts. By encoding quantum states into a
decoherence-protected subspace of two nuclear spins we show that quantum
coherence can be maintained for over 1000 repetitions of the remote entangling
protocol. These results and insights pave the way towards remote entanglement
purification and the realisation of a quantum repeater using NV center quantum
network nodes.

Quantum Physics