Quantum repeaters for secure communication

A communications network based on a quantum-secured IT infrastructure will provide enhanced protection against attacks and improve security for the networking of future quantum computers. Such a network relies on quantum repeaters, which store and transmit data over long distances. Coordinated by Saarland University, partners from both business and science are researching these special repeaters in the QR.N network project. Scientists from the University of Stuttgart are contributing their expertise in quantum physics, photonics, microoptics, and quantum computing to the consortium. The Federal Ministry of Education and Research (BMBF) is funding QR.N with approx. EUR 20 million.

“Quantum communication is a key technology for securing future communications. It protects both IT networks and critical infrastructures from attacks by modern and quantum computers and enables the secure networking of quantum computers,” says Professor Peter Michler, Director of the Institute of Semiconductor Optics and Functional Interfaces (IHFG) at the University of Stuttgart and Deputy Spokesperson of the QR.N research network. Single and entangled photons (i.e., light particles with correlated properties, even when separated by great distances) are used to transmit quantum information through fiber optic cables. They adhere to fundamental physical laws that cannot be altered. Because unknown quantum information units (qubits) cannot be copied, they are particularly well-suited for securing data traffic using new encryption methods.

Because glass fibers absorb photons, particle losses have so far been unavoidable. The photon losses limit the range and reduce the transmission rate. Communication in quantum networks is therefore currently limited to a few hundred kilometers. However, quantum repeaters can offer a solution. They can store quantum information on network nodes, perform complex operations on quantum states, and pass information to the next node, thereby bridging longer distances. In the predecessor project “Quantenrepeater.Link (QR.X)”, the scientists developed the basic components for quantum repeaters. In the follow-up project QR.N., they aim to optimize the components and necessary operations and demonstrate them outside the laboratory. They also want to investigate advanced concepts of quantum repeater technologies such as quantum error correction, new protocols, and multiplexing (i.e., the simultaneous transmission of data over a single channel).

“Quantum repeaters are a crucial component of future quantum networks and one of the key topics in quantum science,” says Michler. Four research groups from the University of Stuttgart are involved in the QR.N network project. The team of Professor Jörg Wrachtrup at the 3rd Institute of Physics will establish a quantum repeater link with up to three network nodes. For a successful demonstration, it must be possible to operate all nodes automatically and maintain long-term stability. The team of Professor Michler at the IHFG will develop ultra-bright single-photon light sources and sources for entangled photon pairs for operation in the telecommunications C-band and integrated into a portable and compact demonstrator for use across various test sections.

The team of Professor Harald Giessen at the 4th Institute of Physics will produce and measure 3D-printed micro-optics for efficiently coupling single-photon emitters (i.e., light sources for single photons), single-photon detectors, and specialized optical fibers. The team of Professor Stefanie Barz at the Institute for Functional Matter and Quantum Technologies (FMQ) will demonstrate how distributed quantum computing functions using particle clusters in photonic circuits.

The “Quantenrepeater.Net (QR.N)” project will run until December 2027. It is funded with approx. EUR 20 million from the Federal Ministry of Education and Research (BMBF) and coordinated by Saarland University. In the QR.N network, 42 partners from research institutions, universities, and commercial enterprises have joined forces to research and test the technology and applications of quantum repeaters in optical fiber networks. The project builds on the results of the BMBF-funded “Quantenrepeater.Link (QR.X)” project, which, under the auspices of Saarland University, researched the foundations for the development of a nationwide quantum repeater from 2021 to the end of 2024. Physicists at the University of Stuttgart have played a major role in shaping both networks. The Institute of Semiconductor Optics and Functional Interfaces (IHFG), the Institute for Functional Matter and Quantum Technologies (FMQ), and the 3rd and 4th Institutes of Physics at the University of Stuttgart are involved in the project.

Project profile on the BMBF website

Press release of Saarland University