(ScientificAmerican) An international team of researchers has built a scalable city-wide quantum network in Bristol, England to share keys for encrypting messages. The network is not capable of connecting eight or more users across distances of 17 kilometers, the demonstration is another milestone toward developing a fully quantum Internet
The network can grow in size without incurring an unreasonable escalation in the costs of expensive quantum hardware. Also, this system does not require any node to be trustworthy, thus removing any security-sapping weak links.
“We have tested it both in the laboratory and in deployed fibers across the city of Bristol” in England, says Siddarth Koduru Joshi of the University of Bristol. He and his colleagues demonstrated their ideas using a quantum network with eight nodes in which the most distant nodes were 17 kilometers apart, as measured by the length of the optical fiber connecting them.
Instead of building a network in which each of the eight nodes is physically connected to all the other nodes, the researchers created one with a central source that sends entangled photons to the eight nodes, named Alice, Bob, Chloe, Dave, Feng, Gopi, Heidi and Ivan. Each node is only connected via a single optical fiber link to the source, making a total of eight links—far less than the 28 that would be required for traditional QKD with no trusted nodes.
So even though the nodes are not physically connected, the protocol the researchers developed establishes a virtual link between each pair of them via the magic of quantum entanglement such that each pair can create a private key.
Future large-scale quantum networks will have to solve at least two major problems: One is that they must interconnect an arbitrarily large number of users. Secondly, such networks have to span vast intracontinental and intercontinental distances—something that requires using either quantum repeaters to extend the range over which one can distribute quantum states or satellites to beam down qubits or entangled particles to nodes on the ground.
Ronald Hanson of Delft University of Technology in the Netherlands, who was not involved in the new work, acknowledges that it extends QKD to “reach many more users within the limited range of QKD without repeaters.”

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