A consortium of Dutch and French researchers said on Wednesday they had maintained quantum entanglement between nodes in Delft and Paris across roughly 900 kilometers of standard telecom fiber — the longest such link yet demonstrated, and the first to run stably for a full hour.
The experiment used a chain of five quantum repeaters installed in existing exchange buildings along the route, each correcting for photon loss without measuring — and therefore destroying — the entangled state.
The team said the result turns long-distance quantum networking into "an engineering problem rather than a physics problem," though transmission rates remain far too low for practical use. A Delft–Brussels–Paris pilot network carrying cryptographic keys is planned for 2028.