A rudimentary quantum network connection between Dutch cities

People can share information (bits) worldwide via the internet. A future quantum internet will enable the sharing of quantum information (qubits) via a new type of network. Such qubits can take not only the values ​​0 or 1, but also superpositions thereof (0 and 1 at the same time). Furthermore, qubits can be entangled, meaning they share a quantum connection, allowing direct correlations regardless of distance.

Researchers around the world are working to build quantum networks that leverage these features to provide fundamentally new communications and computing capabilities beyond today’s Internet. For example, qubits can generate secure encryption keys for securely sharing financial or medical data. Quantum links can also connect quantum computers remotely, increasing their power and giving users access with complete privacy.

Moving out of the laboratory

An international team led by Ronald Hanson of QuTech – a collaboration between TU Delft and TNO – has been 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 that such quantum processors are connected in different cities.”

A few years ago, the team reported the first multi-node quantum network in the laboratory. “We faced new major challenges in moving from these laboratory experiments to realizing a quantum connection between cities. We had to design a flexible system that allows the nodes to operate independently over long distances, we had to consider the impact of photon loss on the connection speed, and we had to ensure reliable confirmation every time the entanglement link was successfully established. Without these innovations, such a long distance would not have been possible.

‘It’s like holding the moon at a constant distance’

To address the challenge of photon loss, the team established the quantum connection using a photon-efficient protocol that required high-precision stabilization of the connecting fiber optic link. Co-author Arian Stolk explains using an analogue: ‘The connection had to be stable within the wavelength of the photons (smaller than a micrometer) over 25 kilometers of optical fiber. That challenge can be compared to maintaining the distance between the Earth and the moon. constantly with an accuracy of just a few millimeters. Through a combination of research insights and applied engineering, we were able to solve this puzzle.”

“In this work, we demonstrate successful entanglement between two quantum network nodes containing diamond spin qubits. The independently operated nodes are connected via a midpoint station via optical fiber. We were able to reliably deliver a pre-specified entangled state between the nodes.”

Collaboration between academia and industry

Co-author Kian van der Enden explains how indispensable the team’s broad expertise was for the success of the project: “Fraunhofer ILT developed a crucial component for this demonstration, a new type of quantum frequency converter. OPNT supplied state-of-the-art timing hardware, Element Six supplied the synthetic diamond materials and Toptica developed lasers with high stability. Finally, the Dutch telecom provider KPN provided the fiber optic infrastructure and the locations of the nodes, the hub and the interchange in The Hague. “

Solid foundation for European quantum internet

This result is an important milestone that addresses key scaling challenges for future quantum networks. Jesse Robbers, Director of Industry & Digital Infrastructure at Quantum Delta NL who co-funded the research, adds: “We continue to show leadership in developing the future foundation of our Digital Infrastructure and how to make it applicable, which is the core of the national and European strategy.”

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