Press release | 24 January 2023 | wrt

Secure Quantum Network in Berlin

tubLAN Q.0 joint project at TU Berlin marks the beginning of a secure communications network in the capital city

Funded with 2.4 million euros by the Federal Ministry of Education and Research (BMBF), a new joint project “tubLANn Q.0” aims to develop a quantum-secured network in the heart of Berlin, with TU Berlin as the starting point. In an initial test, two different connections are to be laid across Campus Charlottenburg starting from the Eugene Paul Wigner Building (EW) on Hardenbergstraße: a fiber optic line to the Main Building on Straße des 17. Juni and a free-beam transmission via air to the TU-Hochhaus (Telekom building) on Ernst-Reuter-Platz. The method used, in which secret keys are exchanged between the sender and receiver via individual light particles, is absolutely secure in theory due to the principles of quantum physics. tubLAN Q.0 aims to use technical innovations to realize this theoretical security in practical applications

Potential disaster for data privacy: In the future, powerful quantum computers capable of easily cracking our current encryption systems could create a worst-case scenario for data protection. In response to this threat, researchers worldwide are working flat-out on developing quantum cryptography, a science that exploits quantum mechanical properties to achieve inherent security. The basis for quantum cryptography, as it is applied here, are individual light particles (photons), which are actually small packets of light waves. These can be used to transmit digital codes made up of zeros and ones, which can be used to fully secure messages.

About the tubLAN Q.0 joint project

Association coordinator: Technische Universität Berlin

Working group of Prof. Dr. Christian Schneider - University of Oldenburg
Working group of Prof. Dr. Martin Siles - University of Applied Sciences Emden / Leer
Working group of Dr. Falk Eilenberger - University of Jena
Entropy GmbH (associated)
PicoQuant GmbH (associated)
Quantum Optics Jena GmbH (associated)
Bundesdruckerei Gruppe GmbH (associated)
DB Systel GmbH (associated)

Funding volume: 2.45 million euros from the BMBF

Duration: until 06/2025

Message is fully encrypted in the key

To achieve this, researchers utilize wave packets which oscillate in different directions with respect to their propagation direction, for example horizontally or vertically to it. If you define one oscillation direction as 0 and the other as 1, it is possible to transmit keys of zeros and ones with the help of the photons. Using such a key, the sender overlays the actual message - if the key is as long as the message, the message is completely hidden in the key and can never be deciphered. Only the receiver can then decode the message with the previously received key.

Implementing the method is extremely complex

There are two particular features to this process: First, for quantum mechanical reasons, the keys are purely random, making the message’s encoding 100 percent secure. Most importantly, when the keys are transmitted, an interceptor on the line can be exposed, because according to the rules of quantum mechanics, the interceptor would change the photons when measuring them. “The process is actually extremely complex,” explains Dr. Tobias Heindel, head of the BMBF-funded junior research group Quantum Communication Systems at TU Berlin. For instance, not only vertical and horizontal but also oscillation directions rotated by 45 degrees are used, each of which can be set and measured with suitable filters. How the filters are used and how the transmitter and receiver exchange information about individual measurement results in order to expose an interceptor is specified in protocols.

Closing technical side-channels to hackers

"There is often a gap between these secure IT protocols and their implementation with real-world measurement devices, as assumptions made in the protocols cannot be completely fulfilled by the technology," says Heindel. "This opens the door to so-called side-channel attacks, in which attackers exploit precisely this gap." In tubLAN Q.0, these dangers are now to be minimized when keys are exchanged over optical fibers between two locations A and B by introducing a central station to which both A and B send photons. These are measured together in the central station and A and B receive the measurement results via normal, classical lines. In combination with the directions of oscillation of the photons sent, which are only known at A and B, the two parties can then generate a secure key. “The amazing thing about this process is that the central station doesn’t have to be especially protected, because a spy cannot do anything with the values measured there without the information available at A and B,” emphasizes Heindel.

Implemented for the first time worldwide

Between the Main Building as the central station and two rooms in the Eugene Paul Wigner building as A and B, this method, also called "measurement-device-independent quantum key distribution" (MDI-QKD), is now to be used for the first time worldwide with spatially separated quantum light sources. The central architecture makes it ideal for an urban network with many different participants who all want to communicate with each other. “In the future, major cities could be connected via satellites using such urban quantum networks,” says Heindel. To achieve this, the researchers hope to gain valuable experience from the optical transmission via air from the Eugene-Paul-Wigner building to the TU-Hochhaus.

Rapid process with new light sources

tubLAN Q.0 is also breaking new ground in terms of the light sources used to emit the photons. All quantum networks realized to date, such as those in Vienna, Cambridge (England), or Hefei (China), use lasers for this purpose. However, these do not emit the required single photons in all cases, but often groups of two or more. These cannot then be used to produce the quantum key, though, and thus reduce the transmission rate. "Since, after all, the keys used have to be just as long as the message itself, this slows down the process noticeably," says Heindel. "We will therefore, for the first time in a realistic network environment, use pure single-photon sources that actually emit only a single light particle at a time."

The research group headed by Prof. Dr. Stephan Reitzenstein at TU Berln is developing the single photon sources for the fiber optic network using the MDI-QKD method. For the open-air transmission, sources are being built in a collaboration between the University of Oldenburg, the University of Applied Sciences Emden / Leer, and the University of Jena. The simulation and theoretical modeling of the experimental results is carried out in collaboration with the junior research group led by Dr. Anna Pappa at TU Berlin, which is funded by the Emmy Noether Program of the German Research Foundation (DFG)