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.
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.
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