Quantum communications

Quantum communications: light particles safeguard networks

A plane took off in March to pursue a very different quantum experiment. This was done for the third in a series of key experiments by the QuNET initiative. Its objective is to ensure secure exchanges of information between government agencies. A team drawn from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF, the Fraunhofer Institute for Telecom­munications, Heinrich-Hertz-Institut, HHI, the German Aerospace Center (DLR), the Max Planck Institute for the Science of Light (MPL), and Friedrich-Alexander-Uni­versität Erlangen-Nürnberg is studying new possibilities toward that aim. Amid today’s geopolitical tensions and the rise of technologies such as quantum computers that can crack existing encryption methods, this is now more important than ever for nation­al security reasons. In addition to government agencies, banks and large data centers could also benefit from the new possi­bilities opened up by quantum communication, using it as an additional means of encrypting sensitive data.

To safeguard existing com­munication networks through quantum key distribution (QKD), the researchers are uti­lizing quantum states that cannot be read out or copied unnoticed. Light pulses gener­ated by a quantum source can be exchanged between two places as keys known only to the sender and the recipient. Any attempt at tampering or eavesdropping would be detected immediately. The pho­tons can either be transmitted via “dark fiber” networks or, to traverse distances of 200 kilometers or more, as a free-space beam sent from satellites to the Earth, for example − or even a combination of both.

“We demonstrate the technological advances that have been made so far in various key experiments that build on each other with increasing complexity,” explains Dr. Thorsten Goebel, who brings together the QuNET activ­ities at Fraunhofer IOF in the role of group manager. The research team started out four years ago with a simple point-to-point connection between two buildings in Bonn. Last year, a pathbreaking experiment in the Berlin area demonstrated that it is possible to use quantum security to encrypt official communica­tions in a complex urban net­work with multiple users. To achieve this, six network nodes at locations operated by Fraunhofer HHI, Deutsche Telekom and Bundesdruckerei, the entity that produces German banknotes and secure docu­ments such as IDs, were linked together with 125 kilometers of optical fiber and additional free-space link connections. Trans­mission of sample ID data be­tween different government agencies and residents was used as a practical use case with re­al-world application. “We were able to show that our systems work stably together with­in heterogeneous network structures and that critical interfaces work to encrypt the exchange of personal data with photons,” Goebel says. Interfaces are the biggest challenge, both between free space and fiber and between systems. “It is crucial for us to keep the conversation about standardization of interfaces in the European context going.”

In the next key experiment, the team plans to demon­strate how a quantum-secured communication connec­tion between a flying airplane and a mobile ground station can be established and ultimately linked to a fiber optic network. In the long run, the flying photons are additionally to be transmitted to an ion trap that serves as stationary quantum memory. The researchers hope this will allow them to expand the range of fiber-bound quantum communication in the future without there being any need to read out the data at interim stops, which could prove to be an issue in terms of security. A mobile setup could be used to close gaps in fiber networks in the future, for example guarding security-relevant events such as a G7 summit in the Alps against eavesdropping with an ad hoc free-space connection. The mobile ground station, which is the size of a small shipping container, uses a periscope to capture the signals from the quantum source on the plane. A telescope stabilizes and combines them so they can be forwarded.

Instead of a plane, a satellite could cover even longer distances down the road. Fraunhofer IOF is now building an optical ground station on top of its new building in Jena to help with future research on satellite connections for quantum communication. The station is to house a stationary receiving telescope. This will enable in-house tele­scope development and function as one element in a global quan­tum network. But investment by the research sector alone will not be enough: “To advance quantum communication, we need to move things through to applica­tion quickly,” Goebel says. “Fed­eral agencies could not only provide funding to support this but also lead by example as early adopters.” Time is pressing, since the potential threat posed by cyberattacks and eavesdrop­ping is growing with advances in technology, including quan­tum computers. And much like quantum computers, quantum encryption is also unlikely to be used on its own in the future: “To achieve maximum security and make systems more resilient, it is a good idea to pursue hybrid approach­es in data encryption and combine quantum key distri­bution with post-quantum cryptography.”