Quantum link set up between Delft and The Hague

An international research team led by Qutech has demonstrated a network connection between quantum processors over metropolitan distances. The team developed fully independently operating nodes and integrated these with deployed optical internet fiber, enabling a 25 km quantum link. The result marks a key advance from early research networks in the lab toward a future quantum internet, which enhances the power of quantum computers as well as improves the privacy of users.

The team led by Ronald Hanson at Qutech was able to connect two small quantum computers between the Dutch cities of Delft and The Hague. “The distance over which we create quantum entanglement in this project, via 25 km of deployed underground fiber, is a record for quantum processors,” says Hanson. “This is the first time such quantum processors in different cities are connected.”

A few years ago, the team reported the first multi-node quantum network inside the lab. “We were faced with new major challenges in going from these lab experiments to realizing a quantum link between cities. We had to design a flexible system that lets the nodes work independently over long distances, we needed to mitigate the impact of photon loss on the connection speed and we had to ensure reliable confirmation each time the entanglement link was successfully created. Without these innovations, such a large distance wouldn’t have been possible.”

To tackle the challenge of photon loss, the team established the quantum connection using a photon-efficient protocol that required very precise stabilization of the connecting fiber link. This necessitates keeping the 25 km link stable to well within the wavelength of the photons. That’s equivalent to keeping the distance between Earth and the moon constant with an accuracy of only a few millimeters.

The architecture and methods are directly applicable to other qubit platforms, including the next-generation scalable qubits the team is currently developing. The successful use of deployed, conventional internet infrastructure sets the stage for a new phase on the road toward a quantum internet. Hanson: “This work marks the crucial step out of the research lab into the field, enabling exploration of first quantum processor networks at metropolitan scale.”