In Germany, researchers successfully transmitted quantum information over a distance of 254 km using commercial optical fiber communication networks. This case, implemented in a real-world setting rather than in a laboratory, is being praised as a step closer to achieving an unhackable "quantum internet."
An international joint research team led by Toshiba Europe published a paper on the 24th in the international journal "Nature."
Quantum communication is a technology that utilizes the principles of quantum mechanics to transmit information encoded in individual particles of light. Among these, "quantum key distribution (QKD)" is a method that allows safely exchanging the secret keys needed for encoding and decoding information.
Due to the characteristics of quantum mechanics, the moment someone attempts to eavesdrop or hack, the state of the light particles changes, resulting in high security. However, conventional optical fiber-based quantum key distribution struggled to maintain signals over distances beyond 100 km as the light signals weakened with increased distance.
To extend the communication distance, the research team focused on a method called twin-field quantum key distribution (TF-QKD). Instead of directly exchanging signals between the sender and receiver, this method employs a relay in between to send signals separately and then combine the information at the relay. This reduces signal loss and enables accurate communication over long distances.
Based on this, the research team developed a quantum communication system over a 254 km commercial optical fiber communication network connecting the cities of Frankfurt, Kell, and Kirchfeld in Germany.
However, there was a challenge that had to be resolved for the communication experiment to succeed. The cornerstone of quantum communication is ensuring that the light sent from two distant points vibrates in sync (in phase). This is referred to as "coherence," and if even a slight discrepancy occurs in the timing of the light, it results in higher errors, making it difficult to generate secret keys or weakening security.
In particular, using a commercial communication network means that numerous signals are transmitted simultaneously, which can interfere with light signals, and the timing of the light can slightly deviate due to temperature changes in the fiber optic or vibrations. To address this issue, the research team ensured that the central relay sends a reference light to the two senders. Each sender synchronized their light signals precisely to this reference light. The adjusted signals are then relayed back, and the relay analyzes the two lights to generate the encryption keys.
At the same time, the team also reduced the expense of quantum communication. To perform quantum communication, superconducting sensors that detect individual particles of light are required. The challenge is that these devices operate only at extremely low temperatures, close to minus 270 degrees Celsius, necessitating expensive cooling systems. The researchers replaced them with semiconductor sensors that operate at around minus 30 degrees. This made the equipment's price and size suitable for commercialization at about one-hundredth and one-tenth of the original levels, respectively.
The system they built maintained a very low error rate even after continuous communication for more than 7 hours a day. The researchers noted, "Secret keys were generated at a speed of about 110 bits per second," adding, "This level of security communication speed is practically usable."
Kim Yong-soo, head of the Quantum Technology Research Division at the Korea Institute of Science and Technology (KIST), explained the significance of the research, stating, "While the communication speed is not higher compared to existing research, it is meaningful as a result achieved in an environment with long distances and significant signal loss."
He emphasized, "China implemented long-distance quantum key distribution exceeding 2000 km, but it was mainly satellite-based and used mostly cryogenic cooling devices. This research has realistically accelerated the commercialization of quantum communication that operates within commercial communication networks."
Quantum communication is a starting point and an essential condition for building a quantum internet that transcends the limitations of the existing internet. The quantum internet is an ultra-fast and ultra-secure communication network that combines quantum communication, quantum networks, and quantum computer technologies. It is expected to be useful in fields where security is critical, such as defense, finance, and healthcare.
The research team stated, "This experiment adopted a 'star-shaped' network structure connecting multiple senders centered around a central relay," looking ahead to the future, stating, "By simply adding senders, the network can be easily expanded, making it suitable for building a national-level quantum communication network, and further enabling the construction of quantum internet across Europe and globally."
References
Nature (2025), DOI: https://doi.org/10.1038/s41586-025-08801-w