Milestone on the road to the ‘quantum internet’

(Press-News.org) Everyday life on the internet is insecure. Hackers can break into bank accounts or steal digital identities. Driven by AI, attacks are becoming increasingly sophisticated. Quantum cryptography promises more effective protection. It makes communication secure against eavesdropping by relying on the laws of quantum physics. However, the path toward a quantum internet is still fraught with technical hurdles. Researchers at the Institute of Semiconductor Optics and Functional Interfaces (IHFG) at the University of Stuttgart have now made a decisive breakthrough in one of the most technically challenging components, the ‘quantum repeater’. They report their results in Nature Communications (DOI: 10.1038/s41467-025-65912-8).

Nanometer-sized semiconductor islands for information transfer

“For the first time worldwide, we have succeeded in transferring quantum information among photons originating from two different quantum dots,” says Prof. Peter Michler, head of the IHFG and deputy spokesperson for the Quantenrepeater.Net (QR.N) research project. What is the background? Whether WhatsApp or video stream, every digital message consists of zeros and ones. Similarly, this also applies to quantum communication in which individual light particles serve as carriers of information. Zero or one is then encoded in two different directions of polarization of the photons (i.e., their orientation in the horizontal and vertical directions or in a superposition of both states). Because photons follow the laws of quantum mechanics, their polarization cannot always be completely read out without leaving traces. Any attempt to intercept the transmission would inevitably be detected.

Making the quantum internet ready for the fiber-optic infrastructure

Another challenge: An affordable quantum internet would use optical fibers—just like today’s internet. However, light has only a limited range. Conventional light signals, therefore, need to be renewed approximately every 50 kilometers using an optical amplifier. Because quantum information cannot simply be amplified or copied and forwarded, this does not work in the quantum internet. However, quantum physics allows information to be transferred from one photon to another as long as the information stays unknown. This process is referred to as quantum teleportation.

Quantum repeaters as nodes for information transmission

Building on this, physicists are developing quantum repeaters that renew quantum information before it is absorbed in the optical fiber. They are to serve as nodes for the quantum internet. However, there are considerable technical hurdles. To transmit quantum information via teleportation, the photons must be indistinguishable (i.e., they must have approximately the same temporal profile and color). This proves extremely difficult because they are generated at different locations from different sources. “Light quanta from different quantum dots have never been teleported before because it is so challenging,” says Tim Strobel, scientist at the IHFG and first author of the study. As part of QR.N, his team has developed semiconductor light sources that generate almost identical photons. “In these semiconductor islands, certain fixed energy levels are present, just like in an atom,” says Strobel. This allows individual photons with defined properties to be generated at the push of a button. “Our partners at the Leibniz Institute for Solid State and Materials Research in Dresden have developed quantum dots that differ only minimally,” says Strobel. This means that almost identical photons can be generated at two locations.

Information is “beamed” from one photon to another

At the University of Stuttgart, the team succeeded in teleporting the polarization state of a photon originating from one quantum dot to another photon from a second quantum dot. One quantum dot generates a single photon, the other an entangled photon pair. “Entangled” means that the two particles constitute a single quantum entity, even when they are physically separated. One of the two particles travels to the second quantum dot and interferes with its light particle. The two overlap. Because of this superposition, the information of the single photon is transferred to the distant partner of the pair. Instrumental for the success of the experiment were “quantum frequency converters”, which compensate residual frequency differences between the photons. These converters were developed by a team led by Prof. Christoph Becher, an expert in quantum optics at Saarland University.

Improvements for reaching considerably greater distances

“Transferring quantum information between photons from different quantum dots is a crucial step toward bridging greater distances,” says Michler. In the Stuttgart experiment, the quantum dots were separated only by an optical fiber of about 10 m length. “But we are working on achieving considerably greater distances,” says Strobel. In earlier work, the team had shown that the entanglement of the quantum dot photons remains intact even after a 36-kilometer transmission through the city center of Stuttgart. Another aim is to increase the current success rate of teleportation, which currently stands at just over 70%. Fluctuations in the quantum dot still lead to slight differences in the photons. “We want to reduce this by advancing semiconductor fabrication techniques,” says Strobel. “Achieving this experiment has been a long-standing ambition — these results reflect years of scientific dedication and progress,” says Dr. Simone Luca Portalupi, group leader at the IHFG and one of the study coordinators. “It’s exciting to see how experiments focused on fundamental research are taking their first steps toward practical applications.”

Quantenrepeater.Net research network

Research into quantum repeaters is funded by the Federal Ministry of Research, Technology and Space (BMFTR) as part of the “Quantenrepeater.Net (QR.N)” project. Coordinated by Saarland University, the QR.N consortium brings together 42 partners from research institutions, universities, and industry to explore and test quantum repeater technology in optical fiber networks. The project builds on the results of the BMFTR-funded (former known as BMBF) “Quantenrepeater.Link (QR.X)” initiative, which, led by Saarland University, laid the groundwork for developing a nationwide quantum repeater between 2021 and 2024. Physicists at the University of Stuttgart have played a major role in shaping both research consortia. The experiments on quantum teleportation were conducted under the coordination of the Institute of Semiconductor Optics and Functional Interfaces (IHFG) at the University of Stuttgart, with contributions from the Leibniz Institute for Solid State and Materials Research (IFW) in Dresden and the Quantum Optics research group at Saarland University.

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