Engineers from Qutech have demonstrated a scalable method to distribute quantum keys over fiber networks. To achieve this goal, they built a measurement-device independent (MDI) system, in which multiple users are connected via a central node that works like a typical telephone switchboard operator. The central node doesn’t need to be trusted: hacking attacks against it cannot break the security of the protocol.
The system consists of three standard telco racks, each in a different city in the Netherlands. The first user connected to the demo setup, code-named Alice, resides in Delft. The second user, called Bob, sits in a KPN building in The Hague. The central node, Charlie, is located in Rijswijk.
To set up untappable quantum communication between Alice and Bob, they both send qubits to Charlie. This generates a detection pattern at the central node, from which a quantum key can be derived to encrypt communication between Alice and Bob. Due to the quantum nature of this key, any eavesdropping will be detected, informing the users that their connection isn’t secure. Neither Alice’s nor Bob’s data is ever available nor known to Charlie.
It’s possible to connect many more users to a central node: unlike currently commercially available quantum key systems, Qutech’s principle scales well. Central nodes can also function as quantum signal repeaters, facilitating quantum communication between different metropolitan networks.