A group of German scientists at Leibniz University of Hannover, the University of Stuttgart, and other researchers has achieved the first intercity QKD experiment with a deterministic single-photon source. The study was published in Light: Science & Application. It will transform the way we defend private data from online threats.
Image Credit: Light: Science & Applications (2024). DOI: 10.1038/s41377-024-01488-0
Conventional encryption techniques are based on the limitations of current computing power and intricate mathematical algorithms. However, as quantum computers become more prevalent, these techniques become more vulnerable, which calls for quantum key distribution (QKD). Using the special qualities of quantum physics, QKD technology enables secure data transfer.
The limitations of current quantum light sources have made it difficult to establish large networks despite years of continuous optimization of this method.
Semiconductor quantum dots (QDs) have enormous potential for lighting quantum light in quantum information technologies. In the quantum world, QDs are known as artificial atoms.
The study involved a group of German scientists led by Professor Fei Ding from Leibniz University of Hannover (LUH), Professor Stefan Kück from Physikalisch-Technische Bundesanstalt (PTB), Professor Peter Michler from the University of Stuttgart, and other collaborators.
This discovery makes semiconductor single-photon sources viable for a practical, secure, long-distance quantum internet.
We work with quantum dots, which are tiny structures similar to atoms but tailored to our needs. For the first time, we used these ‘artificial atoms’ in a quantum communication experiment between two different cities. This setup, known as the ‘Niedersachsen Quantum Link,’ connects Hannover and Braunschweig via optical fiber.
Fei Ding, Professor, Leibniz University of Hannover
The intercity experiment is carried out in the German federal state of Niedersachsen, where Physikalisch-Technische Bundesanstalt (PTB) Braunschweig and Leibniz University of Hannover (LUH) are connected by a deployed fiber that is approximately 79 km in length. Alice at the LUH prepares encrypted single photons statically in polarization.
Bob, who works at the PTB, uses a passive polarization decoder to decode the polarization states of single photons received via fiber-based quantum channels. Moreover, this is Lower Saxony, Germany's first quantum communication link.
The researchers have succeeded in transmitting secret keys quickly and reliably.
First, they confirmed that positive secret key rates (SKRs) can be attained in the lab over distances of up to 144 km, or 28.11 dB loss. Based on this deployed fiber link, a high-rate secret key transmission with a low quantum bit error ratio (QBER) was guaranteed for 35 hours.
Comparative analysis with existing QKD systems involving SPS reveals that the SKR achieved in this work goes beyond all current SPS based implementations. Even without further optimization of the source and setup performance, it approaches the levels attained by established decoy state QKD protocols based on weak coherent pulses.
Dr. Jingzhong Yang, Study First Author, Leibniz University of Hannover
The researchers hypothesize that QDs will also present a great opportunity to realize other quantum internet applications, such as distributed quantum sensing and quantum repeaters, due to their inherent ability to store quantum information and emit photonic cluster states. This study's result highlights the possibility of smoothly incorporating semiconductor single-photon sources into practical, large-scale, high-capacity quantum communication networks.
The use of light's quantum properties in quantum communication makes it impossible for messages to be intercepted.
“Quantum dot devices emit single photons, which we control and send to Braunschweig for measurement. This process is fundamental to quantum key distribution,” said Ding. He expressed his excitement about the outcome of this collaborative effort.
Some years ago, we only dreamt of using quantum dots in real-world quantum communication scenarios. Today, we are thrilled to demonstrate their potential for many more fascinating experiments and applications in the future, moving towards a ‘quantum internet’.
Fei Ding, Professor, Leibniz University of Hannover
Journal Reference:
Yang, J., et al. (2024) High-rate intercity quantum key distribution with a semiconductor single-photon source. Light: Science & Applications volume. doi.org/10.1038/s41377-024-01488-0