Scientists overcome limitations of Quantum cryptography for the internet

Scientists overcome limitations of Quantum cryptography for the internet
Technology News |
Until now, quantum cryptographically encrypted communication processes were only possible between exactly two participants. For use in telecommunications networks, this limitation represented a decisive obstacle. Scientists from Austria have now overcome this limitation – in a quantum physics experiment, they succeeded the first time connecting four participants within a quantum network in such a way that each of them could exchange encrypted messages with each other.
By Wisse Hettinga

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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 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.


The photon pairs are generated like in a rainbow, explains Sören Wengerowsky, first author of the study. “This gives us the opportunity to clearly distribute them among the participants. Each participant then measures the polarization of his photon.”

Although the result of this measurement is random for each participant according to the laws of quantum physics, it is always identical for both parts of a photon pair. Thanks to this entanglement, all participants in the network can generate cryptographic keys and use them for tap-proof communication.

A decisive advantage of this architecture is its flexibility, the scientists emphasize. This allows new communication partners to be dynamically integrated into the quantum network with only minimal intervention. In your opinion, this proves that quantum networks can be practically implemented.

In the future, it will be possible to set up comprehensive networks that allow users to communicate with one another in a tap-proof manner, even over long distances – a prerequisite for setting up a quantum Internet. The researchers now want to prove the expandability of the new quantum network architecture in further experiments.

Original article: https://www.nature.com/articles/s41586-018-0766-y

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