Three-Node Quantum Network is a Step Towards the Quantum Internet
(InfoQ.com) Researchers at the Delft University QuTech center created the first multi-node quantum network, which is a step in the direction of building a network of interconnected quantum computers.
The key aspect of this achievement is the presence of the third node acting as a router. In fact, several of the researchers involved in the creation of the first three-node network had previously shown how to entangle two electron spin states over a 1.3 km distance. That essentially provided the basic building block for a communication network, roughly corresponding to the first layer of the ISO/OSI model. But without the possibility of relaying the quantum state through some kind of intermediate node, the distance separating the origin and destination qubits was necessarily going to be limited.
The complexity of adding a relaying node comes from the fact that if you do not know in advance the quantum state you wish to replicate, then creating a copy will destroy the original. To overcome this limitation, the researchers introduced an additional qubit within the middle node acting as a “memory qubit”, which enabled passing the entangled state from the origin to the destination node. Leveraing their previous research, the entanglement of remote qubits was achieved using a system of mirrors and lasers to entangle a photon and beam it from one node to the other.