The quantum internet of the future will enable completely tap-proof communication between users worldwide. Instead of strong light signals of classical communication technology, individual light particles (photons) are used to generate a cryptographic key. This key can then be used to encrypt data and send it by conventional means. If the recipient has the same key, he can decrypt the data. The security of this process is based on a quantum physical law: it is impossible to copy the state of a single light particle without errors. If someone tries it anyway, he causes errors in the transmission, and the access attempt is revealed. Therefore, quantum communication experts are convinced that it is not possible to hack such a communication link. In a conventional data network, on the other hand, information can be copied infinitely often without leaving traces.
In principle, it is known how quantum cryptography can also be used in data networks, explains Rupert Ursin.Rupert Ursin, research group leader at the Institute of Quantum Optics and Quantum Information at the Austrian Academy of Sciences (ÖAW) in Vienna. "So far however, this has only been implemented experimentally with great limitations," continues the co-author of the study published as a cover story in the scientific magazine "Nature". Previous quantum networks could in most cases only connect two participants under guarantee of security. For the interconnection of multiple participants, highly complex and error-prone hardware setups were necessary, which, however, ultimately also allowed only limited communication connections.
The Vienna researchers found a way out of this limitation by designing a new network architecture and putting it to the test in an experiment: They connected four participants in a quantum network and supplied them with individual light particles from a single source. Already at the source, the photons were generated in pairs with an unknown but identical polarization.