The QTF-Backbone: Proposal for a Nationwide Optical Fibre Backbone in Germany for Quantum Technology and Time and Frequency Metrology

Imagine a world where the most precise measurements of time and the most secure ways to send information are locked away in a few specialized laboratories, accessible only to a handful of scientists. For decades, this has been the reality for the field of quantum technology and high-precision metrology. These fields rely on two powerful tools: the ability to send quantum information, which is information carried by individual particles of light that cannot be copied without being detected, and the ability to distribute time and frequency signals with extreme accuracy, far beyond what satellite systems can provide. Currently, these capabilities exist in isolated test lines, like single threads of a tapestry that have not yet been woven together. Without a connected network, researchers cannot test how these technologies behave over long distances or how they might work together to solve real-world problems.

A new proposal seeks to change this by weaving those threads into a single, nationwide fabric. A large group of scientists and engineers from across Germany has put forward a plan to build the QTF-Backbone, a dedicated national network of optical fibers designed specifically to carry quantum signals and ultra-precise time signals. This is not a proposal to upgrade the existing internet, which is designed for moving vast amounts of data quickly, but rather to create a separate, specialized highway for the most delicate and precise signals in science. The authors argue that by connecting research institutions, universities, and industries with this dedicated infrastructure, Germany can move from isolated experiments to a fully functional system that supports the next generation of secure communication, navigation, and fundamental physics research.

The core of the proposal is a plan to lay out a network of "dark fibers"—optical cables that are already buried underground but are not currently carrying any light signals. These fibers will be reserved exclusively for the QTF-Backbone. The researchers envision a network that stretches across the country, connecting key points of presence where scientists can plug in their equipment. The plan is to roll this out in four phases over ten years. It begins with a "pathfinder" link to test the technology, followed by a rapid expansion to connect thirteen major hubs, and then further growth to cover more of the country. By the end of the decade, the goal is to have a robust network spanning nearly 4,700 kilometers, linking dozens of locations. This infrastructure will allow scientists to send signals that are stable enough to compare atomic clocks with a precision that was previously impossible outside of national metrology institutes.

The paper details how this network will work by separating the physical layers of the system. One pair of fibers will carry the quantum signals, which are extremely fragile and require a quiet environment to travel without losing their unique properties. Another pair will carry the time and frequency signals, which act as a master clock for the entire network. These signals will be generated at a central location, likely at the national metrology institute, and distributed to users across the country. Along the way, the network will include special sites where signals are amplified and routed, ensuring they can travel long distances without degrading. The researchers emphasize that this network will be built on "dark" fibers, meaning no other data traffic will interfere with these sensitive signals. This is crucial because even the slightest vibration or temperature change in a busy commercial cable can disrupt the delicate quantum states or the precise timing required for these experiments.

One of the most significant findings of the paper is the potential for this network to revolutionize how we understand and measure the world. With such a precise distribution of time, scientists can compare atomic clocks in different cities with an accuracy that allows them to detect tiny changes in gravity. This capability opens the door to a new way of measuring height and mapping the Earth's surface, known as relativistic geodesy. Instead of relying on traditional surveying methods that can accumulate errors over long distances, scientists could use the slight slowing of time caused by gravity to measure the height of a mountain or the depth of a valley with unprecedented accuracy. The paper suggests that this network could also help validate satellite missions that monitor the Earth's gravity field, providing a ground-based check that improves our understanding of climate change, melting ice sheets, and rising sea levels.

Beyond geodesy, the network is designed to be the testing ground for the future of secure communication. The authors explain that current methods of sending secure messages over long distances are limited by the distance the signal can travel before it fades. To overcome this, the network will allow researchers to test quantum repeaters, devices that can extend the range of quantum signals without destroying them. This is a critical step toward building a quantum internet, a future network where information is transmitted with absolute security. The paper notes that while some of these technologies are still in the experimental stage, having a dedicated national network will accelerate their development by allowing researchers to test them in real-world conditions rather than just in controlled laboratories.

The proposal also addresses the resilience of our critical infrastructure. Today, many essential systems, from power grids to financial markets, rely on satellite signals for timing. These signals can be disrupted by jamming, spoofing, or even natural events. The QTF-Backbone offers a terrestrial backup that is immune to these threats. By distributing time signals through underground fibers, the network provides a reliable way to keep systems synchronized even if satellite access is lost. The paper highlights that this is not just a theoretical benefit but a practical necessity for national security and economic stability. The network would allow government labs and timing centers to test these backup systems on a large scale, ensuring that the country remains connected and operational in the face of potential disruptions.

The authors are clear that this is a proposal, not a finished project. They outline a detailed funding concept, estimating that the initial ten-year period will require significant investment from the government to cover the costs of renting the fibers and hiring the necessary staff. They propose that after this initial phase, the network could become self-sustaining through membership fees from the institutions that use it, similar to how other research networks operate. The paper also details the governance structure, suggesting a non-profit association will manage the network, with input from a scientific advisory board to ensure it meets the needs of the research community. This structure is designed to ensure that the network remains a public resource for science and innovation, rather than a commercial product.

The paper concludes by positioning this project as a vital step for Germany and Europe. By building this infrastructure, the country aims to become a central hub for quantum technology and precision metrology, fostering collaboration between academia, industry, and government. The authors believe that the QTF-Backbone will not only advance scientific knowledge but also create new markets and economic opportunities. It will provide a platform for startups and small businesses to develop new technologies, from secure communication devices to advanced sensors. Ultimately, the proposal is a call to action to invest in the physical infrastructure that will support the scientific breakthroughs of the future, ensuring that the benefits of these technologies are accessible to all, not just a select few.