Providing for quantum communication over longer distances using fiber optic networks

Ever-increasing computing power and the prospect of the availability of quantum computers mean that current encryption techniques could become vulnerable. Assuming that a quantum computer is specifically programed for code breaking, it could easily snoop on ordinary computers using standard protocols.

However, if encryption keys are exchanged in the form of light particles, so-called photons, the laws of physics guarantee that any attempt at hacking would be discovered. "If quantum communication is to become a viable future technology, we need to ensure it also operates reliably over long-distance fiber optic networks covering larger areas," emphasized Professor Christoph Becher, Professor of Experimental Physics and head of the Quantum Optics Group at Saarland University.

Becher is coordinating an association of 43 partners from the fields of science and industry on one of the greatest technological challenges, i.e., the development of quantum repeaters and their integration into existing fiber optic networks. Unavoidable link limitations mean that quantum communication is currently restricted to distances of a few hundred kilometers.

It is not possible to overcome these limitations by means of signal amplification as is the case with conventional optical fiber communication methods. Instead, quantum repeaters will enable communication over longer distances by breaking down information into smaller linked pieces using quantum processes.

Field testing outside the protected lab environment planned

The QuantumRepeater.Link network is based on the Q.Link.X project, in which researchers were able to produce important basic components of quantum repeaters. In the new project, these components are to be optimized and integrated in fiber optic test networks outside protected lab environments.

The main objective is to demonstrate that an elementary quantum repeater system can be made to function successfully over distances of up to 100 kilometers. Various promising approaches based on the use of a range of suitable materials will be tested and developed as appropriate. The network hopes to overcome technological obstacles and make the serial production of quantum repeaters a viable option for the future. The research network was officially launched on August 1, 2021.

The research network consists of 43 partners from university research institutions, commercially-linked institutes as well as various businesses, including Deutsche Telekom. The collaboration with business partners and an external advisory board will allow to evaluate results in terms of their feasibility. The achievements of the consortium are to be secured over the long term by means of patent applications and the targeted support of spin-offs.

JGU subproject concerns theoretical modeling and experiments

Researchers at Johannes Gutenberg University Mainz will be considering quantum communication from the experimental and theoretical perspectives. "Our experiments will be concerned with a specific potential platform, namely what is known as crystallographic defects in diamond," explained Professor Peter van Loock and Professor Ferdinand Schmidt-Kaler of the JGU Institute of Physics. Peter van Loock heads up the JGU subproject and is responsible for the theoretical research, while Ferdinand Schmidt-Kaler is in charge of the experimental aspects.

The diamond silicon-vacancy color centers that will be the subject of experimentation are characterized by narrow bandwidth light emission that is to be used for the spatial distribution of quantum mechanical entangled states by means of single-photon emission and detection. In addition to the color centers, atoms, ions and semiconductor systems will also be investigated within the joint project for their relevance to quantum repeater functioning.

The theoretical project at JGU will play a leading role in the theoretical modeling of all experimental quantum repeater platforms. However, new theoretical concepts involving the hybridization of various hardware platforms or incorporating methods of quantum error correction in the repeater protocols will also be explored.