The Internet allows people to share information (bits) globally. A future quantum internet will allow quantum information (qubits) to be shared through a new type of network. Such qubits can take not only the values ​​0 or 1, but also their overlaps (0 and 1 at the same time). Additionally, qubits can be entangled, meaning they share a quantum connection that allows for instantaneous correlations regardless of distance.

Researchers around the world are working to build quantum networks that use these features to provide fundamental communication and computing capabilities that coexist with today’s Internet. For example, qubits can generate secure encryption keys for securely sharing financial or medical data. Quantum links can also connect distant quantum computers, increasing their power and allowing access with complete privacy for users.

Moving out of the lab

An international team led by Ronald Hanson at QuTech — a collaboration between TU Delft and TNO — has succeeded in connecting two small quantum computers between the Dutch cities of Delft and The Hague. “The distance we are creating quantum crossover in this project, via 25km of deployed underground fiber, is a record for quantum processors,” says Hanson. “This is the first time such quantum processors from different cities have been connected.”

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

“Like keeping 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 very precise stabilization of the connecting fiber link. Co-author Arian Stolk explains using an analogy: “The link had to be stable in the photon wavelength (less than a micrometer) over 25 kilometers of optical fiber. This challenge is compared to keeping the distance between the Earth and the Moon. constant, with a precision of only a few millimeters, through a combination of research and applied engineering perspectives, we managed 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 by a fiber optic midpoint station. We were able to reliably deliver a pre-specified state of entanglement between nodes.”

Collaboration between academia and industry

Co-author Kian van der Enden explains how indispensable the team’s extensive expertise was to the success of the project: “Fraunhofer ILT developed a critical component for this demonstration, a new type of quantum frequency converter. OPNT provided state-of-the-art timing hardware, Element Six provided its synthetic diamond materials, and Toptica developed high-stability lasers. Finally, Dutch telecommunications provider KPN provided the fiber infrastructure as well as node locations, the midpoint and the hub in The Hague. “

A solid foundation for the European quantum internet

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

The architecture and methods are directly applicable to other qubit platforms, including the next generation of scalable qubits that the team is currently developing. The successful use of deployed conventional Internet infrastructure lays the foundation for a new stage on the road to a quantum Internet. Hanson: “This work marks the crucial step from the research laboratory in the field, enabling the exploration of the first metropolitan-scale quantum processor networks.”