In a recent article published in Light: Science and Applications, a research team from Germany, including Professor Fei Ding from Leibniz University of Hannover (LUH), Professor Stefan Kuck from Physikalisch-Technische Bundesanstalt (PTB), and Professor Peter Michler from the University of Stuttgart, successfully conducted the first intercity QKD experiment using a deterministic single-photon source. This development marks a significant advancement in the field of quantum communication, offering a new method for securing sensitive information against cyber threats.
The experiment utilized semiconductor quantum dots (QDs), often referred to as artificial atoms, to generate quantum light for information technologies. This breakthrough demonstrates the potential of semiconductor single-photon sources for enabling a 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 fibre," explained Professor Fei Ding.
The intercity experiment took place in Niedersachsen, Germany, where a 79-kilometer optical fiber link connects LUH in Hannover with PTB in Braunschweig. In the experiment, the team at LUH, referred to as Alice, prepared single photons with polarization encryption. At PTB, Bob used a passive polarization decoder to decrypt the polarization states of the received photons through the fiber-based quantum channels. This setup also represents the first quantum communication link in Lower Saxony, Germany.
The researchers achieved stable and rapid transmission of secret keys over the network. They confirmed that positive secret key rates (SKRs) are achievable over distances of up to 144 km, corresponding to a 28.11 dB loss in a controlled laboratory setting. Furthermore, they ensured a high-rate transmission of secret keys with a low quantum bit error ratio (QBER) over a continuous 35-hour period using the deployed fiber link.
"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 optimisation of the source and setup performance, it approaches the levels attained by established decoy state QKD protocols based on weak coherent pulses," commented Dr. Jingzhong Yang, the first author of the study.
The team also noted the potential of quantum dots for other quantum internet applications, such as quantum repeaters and distributed quantum sensing, due to their ability to store quantum information and emit photonic cluster states. This work highlights the feasibility of integrating semiconductor single-photon sources into practical, large-scale, and high-capacity quantum communication networks.
The demand for secure communication is as old as civilization itself. Quantum communication ensures that messages remain secure by utilizing the quantum properties of light. "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 enthusiasm about the research collaboration, stating, "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'."
Research Report:High-rate intercity quantum key distribution with a semiconductor single-photon source
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