University of Stuttgart Achieves Breakthrough in Quantum Communication

Researchers at the University of Stuttgart have achieved a significant milestone in quantum communication by successfully teleporting quantum information between photons emitted by separate quantum dots. This breakthrough addresses a major challenge in developing quantum repeaters, essential for extending quantum communication over long distances.

Quantum communication leverages the principles of quantum mechanics to enable secure information transfer. A key component in this field is the quantum repeater, designed to extend the range of quantum communication by mitigating signal loss over long distances. Unlike classical repeaters, quantum repeaters cannot simply amplify signals due to the no-cloning theorem, which prohibits the copying of unknown quantum states. Instead, they rely on processes like entanglement swapping and quantum teleportation to transmit information.

The University of Stuttgart's team, led by Prof. Peter Michler, focused on transferring quantum information between photons from different quantum dots. Quantum dots are nanometer-sized semiconductor structures that can emit single photons with defined properties. The challenge lies in ensuring that photons from separate quantum dots are indistinguishable in terms of temporal profile and color. To achieve this, the researchers developed nearly identical semiconductor light sources and utilized quantum frequency converters to compensate for residual frequency differences between the photons. This setup allowed them to teleport the polarization state of a photon from one quantum dot to another over a fiber link.

This breakthrough was a collaborative effort involving multiple institutions:

• University of Stuttgart: Led the experiment and developed the semiconductor light sources.

• Leibniz Institute for Solid State and Materials Research (IFW) in Dresden: Provided quantum dots with minimal differences, enabling the generation of nearly identical photons.

• Saarland University: Developed the quantum frequency converters essential for synchronizing the photons.

The research was conducted under the "Quantenrepeater.Net" (QR.N) project, a consortium of 42 partners from research institutions, universities, and industry, funded by the Federal Ministry of Research, Technology, and Space (BMFTR).

The successful teleportation of quantum information between photons from different quantum dots represents a pivotal step toward the development of a quantum internet. A quantum internet would enable ultra-secure communication channels, leveraging quantum entanglement to detect any eavesdropping attempts. The ability to transfer quantum states over fiber links using quantum repeaters is crucial for establishing long-distance quantum networks. This experiment demonstrates the feasibility of such repeaters, bringing the vision of a quantum internet closer to reality.

Prior to this achievement, quantum teleportation had been demonstrated using photons from the same source. The challenge of teleporting information between photons from different sources remained unresolved due to difficulties in ensuring photon indistinguishability. This experiment is the first to overcome this hurdle, marking a significant advancement in the field.

While the experiment successfully demonstrated quantum teleportation over a 10-meter fiber link, the researchers aim to extend this distance significantly. Previous work by the team showed that entanglement of quantum dot photons remains intact after a 36-kilometer transmission through Stuttgart's city center. Future efforts will focus on increasing the success rate of teleportation, currently at just over 70%, by advancing semiconductor fabrication techniques to reduce fluctuations in the quantum dots.

The University of Stuttgart's successful teleportation of quantum information between photons from separate quantum dots marks a significant advancement in quantum communication. This achievement not only addresses a longstanding challenge in the field but also paves the way for the development of a secure and practical quantum internet.