Fraunhofer ILT starts operating quantum internet node codeveloped with TNO

04 Jun 2025

German-Dutch group developing components, including frequency converters, lasers, optics, and single photon sources.

Fraunhofer Institute for Laser Technology (ILT) has started operating a quantum internet node that it developed with its Dutch partner TNO. The system is almost identical to the network nodes with which the Dutch research center QuTech demonstrated a quantum entanglement link between The Hague and Delft. The new node will serve as a research platform.

ILT is testing it with partners from industry and science in a local network and continuing to develop the technology. The institute is focusing on photonic components: quantum frequency converters, lasers, optics or single photon sources.

The quantum internet is still in its infancy. It can be used to transport entangled quanta, which should protect data from unauthorized access. QuTech has now entangled two stationary qubits in Delft and The Hague over 25 km of conventional telecoms fiber optics; in other words, it has put them into a joint quantum state.

Photons serve as the transport medium, being inherently entangled with the quantum state of the NV center. By deliberately superimposing these photons, the Dutch researchers were able to transfer the entanglement to the qubits of the network nodes.

“An internet secured by quantum entanglement would enable secure remote access to quantum computers so that the limited quantum hardware available would be accessible to many users,” said Dr. Bernd Jungbluth, head of the Strategic Mission Initiative Quantum Technology at ILT.

Methods based on quantum physics could also be used to ensure that information is transferred anonymously (for example, in whistle-blowing). Blind quantum computing is also considered promising. This allows users to access the computing power of remote quantum computers without their operators being able to view the input data, algorithms or results.

Photonic key components

In quantum internet development, research is mainly focusing on photonic network nodes such as those in The Hague, Delft and now Aachen. At their heart are color center qubits in diamond, known as NV centers, which are nitrogen vacancies in the crystal lattice of diamonds. But other defects are also possible.

Excitation creates an additional electron in the vacancy and, thus, a quantum system whose energy states the researchers can control using laser light, microwaves and electric and magnetic fields. The excited NV center emits a single photon in the visible spectral range on demand, which is entangled with the electron spin in this system, and which can be used for entanglement with other nodes connected via the optical fiber.

However, this photonic transmission reaches its limits over longer distances, but ILT has developed a solution to overcome this limit: An almost noise-free quantum frequency converter shifts the wavelength of the photons into the low-loss telecom spectrum around 1550 nm. “To exchange the superposition states between the network nodes, we have to get as many photons as possible to the other end of the line,” said Jungbluth.

In a project funded by the German state of North Rhine-Westphalia (NRW), his team has now developed the new network node with TNO. In addition to its quantum frequency converter, they are also using optical assemblies they developed. Jungbluth’s team transferred the system to Aachen and installed the node at ILT. Here, the system has been integrated into a local fiber infrastructure.

The institute plans to test the node in the local network and to continue to develop it with partners from industry and science, focusing on key photonic components. Components also include the single photon sources, detectors and the lasers and optics. In cooperation with Dr. Florian Elsen’s junior research group at RWTH Aachen University, the team will also be working on interfaces to other qubit platforms.

Nucleus for a quantum internet

The team in Aachen is creating an open test and development environment for photonic quantum hardware around the node. The platform is not only open to partners from NRW, but also to European research institutions and companies. Their aim is to jointly develop interfaces, protocols and components for the quantum internet of the future, and to keep an eye on their compatibility right from the start.

“Fraunhofer ILT is thus making a concrete contribution to European networking – as is also being pursued by the Quantum Internet Alliance (QIA),” says Jungbluth. Now that the basis has been created with the node technology, there is an opportunity to establish it as a central link in a future European test network.

The start of operations in Aachen also marks a milestone for the quantum technology state of NRW: As the first of its kind, the node can potentially act as a starting point for developing the quantum internet in Germany.

“There are plans to integrate it into a metropolitan-scale quantum network in the future,” said Jungbluth, adding that the initial focus is on testing, optimizing and miniaturizing the photonic building blocks. This work also serves to transfer know-how from basic research to industry, an area in which ILT has specialized for over 40 years.