(PhysicsWorld) Scientists at the Delft University of Technology in the Netherlands have taken an important step towards a quantum Internet by connecting three qubits (nodes) in two different labs into a quantum network. Such quantum networks could be used for secure communication, for safer means of identification or even distributed quantum computing.
Expanding a quantum network from two nodes to three – and, in time, from three to many – is not as simple as just adding more links. The task is complicated by the fact that noise (which can destroy quantum information) and optical power levels vary greatly across the network.
The Delft team addresses this problem using a twofold stabilization scheme. The first, local element of the scheme focuses on stabilizing the interferometers used to generate entanglement between the communication qubit and the “flying” qubit at each node.
The second, global part of the stabilization involves directing a portion of the laser light towards a separate interferometer used to generate entanglement between nodes. The interference is measured and the signal coupled to a fibre stretcher in one arm of the interferometer.
The researchers suggest that their network could be expanded by increasing the number of qubits at a single network node, similar to the ten-qubit diamond NV-centre that a different Delft group created in 2019. They also say that the new network provides a platform for developing higher-level quantum network control layers that would make it possible to automate the network

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