A multiqubit interface for trapped ions and travelling photons*

Quantum networks could enable powerful applications in quantum enhanced distributed sensing, timekeeping, cryptography and multiparty protocols. The most remarkable applications of quantum networks require nodes that are intermediate scale quantum computers that can be coupled (entangled) with photons over long distances. Such nodes thus comprise registers of matter qubits capable of memory and quantum-logic, interfaced with photons. We present a multi-qubit telecom-interfaced quantum network node that consists of a string of electrically-trapped ions, coupled to an optical cavity. We experimentally demonstrate the node's capabilities of photon-matter entanglement distribution over long distance, deterministic quantum logic, and quantum memory. Specifically, first we produce trains of single photons, where each photon is entangled with a different ion and show that the entanglement survives up to at least 100 km distance of photon travel in a telecom fibre.  Second, using deterministic quantum logic gates on ions we produce 2- and 3- qubit entangled states of the travelling photons by entanglement swapping. Third we exploit the memory capability to realize a quantum repeater protocol over a 50 km fibre channel.