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. InfoQ has spoken with Ronald Hanson, principal investigator at QuTech, to learn more.
In their Science paper, the researchers show how it is possible to distribute entangled states across the qubits laid out in three nodes and making up a small quantum network.
We combine remote quantum nodes based on diamond communication qubits into a scalable phase-stabilized architecture, supplemented with a robust memory qubit and local quantum logic.
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.
InfoQ has spoken with Ronald Hanson, principal investigator at QuTech, to learn more.
InfoQ: Could you explain what is the importance of your achievement? What advancement is made possible by your three-node quantum network?
Ronald Hanson: We realized the world’s first multi-node quantum network, that is, a network that connects small quantum processors using entanglement. With this network we can start to test quantum internet protocols at small scale and use it as a development testbed for a quantum network protocols and control stack – similar to the layers of abstraction current internet.
InfoQ: What are the main challenges that you had to overcome to reach this result?
Hanson: We had to invent a new scheme to connect the different processors that would allow us to stabilize the optical phase between each segment of the network (a requirement for making entangled links), we had to improve our quantum memory significantly, and we had to fabricate and control three small quantum processors.
InfoQ: Could you briefly describe what your research is currently focusing on as a "next step” towards quantum networks?
Hanson: Next steps are to expand the network with more qubits and different functionality, and in parallel to test the first quantum internet control layers, in particular the link layer.
While exciting, QuTech research is still in its infancy and a quantum internet is definitely not around the corner. InfoQ will continue to timely cover progress in the field.