Life online stays vulnerable. Criminals can infiltrate bank accounts or steal personal identities, and AI helps these attacks change into more sophisticated. Quantum cryptography offers a promising defense by utilizing the principles of quantum physics to secure communication against eavesdropping. Even so, constructing a functioning quantum web still involves major technical challenges. A team on the Institute of Semiconductor Optics and Functional Interfaces (IHFG) on the University of Stuttgart has now made significant progress on probably the most difficult components, the “quantum repeater.”
Their study appears in Nature Communications.
Quantum Dots as Tiny Platforms for Information Transfer
“For the primary time worldwide, we now have succeeded in transferring quantum information amongst 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. To know why this matters, it helps to have a look at how communication works. Whether someone sends a WhatsApp message or streams a video, the info at all times relies on zeros and ones. Quantum communication follows an analogous idea, but individual photons act as the knowledge carriers. A zero or one is encoded through the direction of the photon’s polarization (i.e., their orientation within the horizontal and vertical directions or in a superposition of each states). Because photons behave in response to quantum mechanics, their polarization can’t be measured without leaving detectable traces. Any try and intercept the message could be exposed.
Preparing Quantum Networks for Fiber Optics
One other critical issue involves compatibility with today’s web infrastructure. A reasonable quantum web would depend on the identical optical fibers used now. Nevertheless, light traveling through fiber might be transmitted only over limited distances. Conventional signals are refreshed roughly every 50 kilometers using an optical amplifier. Quantum information can’t be amplified or copied, which implies this approach doesn’t work. As a substitute, quantum physics makes it possible to transfer information from one photon to a different so long as the knowledge itself stays unknown. This phenomenon is named quantum teleportation.
Developing Quantum Repeaters for Long-Distance Transfer
To reap the benefits of quantum teleportation, scientists are designing quantum repeaters that may renew quantum information before it disappears within the fiber. These repeaters would function as essential nodes in a quantum web. Creating them has been difficult. Teleportation requires the photons to be nearly equivalent in properties corresponding to timing and color. Producing such photons is tough because they arrive from separate sources. “Light quanta from different quantum dots have never been teleported before since it is so difficult,” says Tim Strobel, scientist on the IHFG and first creator of the study.
As a part of QR.N, his team developed semiconductor light sources that emit photons that closely match one another. “In these semiconductor islands, certain fixed energy levels are present, identical to in an atom,” says Strobel. This setup enables the production of individual photons with well-defined characteristics. “Our partners on the Leibniz Institute for Solid State and Materials Research in Dresden have developed quantum dots that differ only minimally,” he adds. This makes it possible to generate nearly equivalent photons in two separate locations.
Teleporting Information Between Photons From Different Sources
On the University of Stuttgart, the researchers successfully teleported the polarization state of a photon from one quantum dot to a photon produced by a second quantum dot. One dot emits a single photon and the opposite generates an entangled photon pair. “Entangled” means the 2 photons share a single quantum state even when physically apart. One photon from the pair travels to the second quantum dot and interacts with its photon. When the 2 overlap, their superposition transfers the knowledge from the unique photon to the far-away partner of the entangled pair.
A key element of this achievement was using “quantum frequency converters,” devices that adjust small frequency mismatches between photons. These converters were designed by a team led by Prof. Christoph Becher, a quantum optics specialist at Saarland University.
Working Toward Longer Distances and Higher Accuracy
“Transferring quantum information between photons from different quantum dots is a vital step toward bridging greater distances,” Michler explains. On this experiment, the 2 quantum dots were linked by about 10 meters of optical fiber. “But we’re working on achieving considerably greater distances,” says Strobel.
Earlier research had already shown that entanglement between quantum dot photons can survive a 36-kilometer transmission through town center of Stuttgart. The team also goals to extend the teleportation success rate, which is currently just a little above 70%. Variations inside each quantum dot still cause small inconsistencies within the photons.
“We would like to cut back this by advancing semiconductor fabrication techniques,” says Strobel. Dr. Simone Luca Portalupi, group leader on the IHFG and certainly one of the study coordinators, adds, “Achieving this experiment has been a long-standing ambition — these results reflect years of scientific dedication and progress. It’s exciting to see how experiments focused on fundamental research are taking their first steps toward practical applications.”
A Nationwide Effort to Construct Quantum Repeater Technology
Research on quantum repeaters receives funding from the Federal Ministry of Research, Technology and Space (BMFTR) as a part of the “Quantenrepeater.Net (QR.N)” project. Coordinated by Saarland University, the QR.N network includes 42 partners from universities, research institutes, and industry who collaborate on developing and testing quantum repeater technology in optical fiber networks. This system builds on results from the sooner “Quantenrepeater.Link (QR.X)” initiative, also supported by the BMFTR (formerly BMBF), which helped lay the inspiration for a nationwide quantum repeater from 2021 to 2024. Scientists on the University of Stuttgart have played a central role in each efforts.
The quantum teleportation experiments were carried out under the leadership of the Institute of Semiconductor Optics and Functional Interfaces (IHFG) with contributions from the Leibniz Institute for Solid State and Materials Research (IFW) in Dresden and the Quantum Optics research group at Saarland University.