Quantum cryptography

Alice and Bob are going to have company. Until now, quantum cryptographic encrypted communication has taken place primarily between two active participants, usually called Alice and Bob. This has been a major shortcoming for use in modern networks, which must allow simultaneous communication between countless active participants.

Researchers at the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, in Vienna, have now overcome this hurdle. In a quantum physics experiment, they worked together with the Austrian Institute of Technology (AIT) to connect four participants within a quantum network, such that each of them could exchange encrypted messages with any other participant. This represents a milestone on the way to a quantum encrypted Internet.
 
Tap-proof communication between multiple users
 
The quantum Internet of the future should enable completely tap-proof communication between users worldwide. Instead of strong light signals used in classical communication technology, individual light particles (photons) are used to generate a cryptographic key. With this, data can then be encrypted and sent using classical techniques. If the recipient has the same key, they can decipher the data. The security of this process is based on a law of quantum mechanics: it is impossible to copy the state of a single light particle error-free. And if somebody tries, it generates errors in the transmission and blows their cover. By contrast, in a conventional network, messages can be copied indefinitely without leaving a trace.
 
How to use quantum cryptography in networks is known in principle, explains Rupert Ursin, quantum physicist and group leader at the Austrian Academy of Sciences. "However, so far, this has only been experimentally implemented with great restrictions," continues the co-author of the study now published in "Nature". In most cases, previous quantum networks were only able to connect two active participants with guaranteed security. By contrast, highly complex and error-prone hardware setups were required to connect several participants, ultimately allowing only limited communication links.
 
New architecture for quantum network
 
The Viennese researchers have now found a way around this restriction 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 from a single source with individual light particles. At the source, the photons were generated in pairs with an unknown, but identical polarization (polarization denotes the oscillation direction of the photons).
 
Sören Wengerowsky, first author of the study, describes the next steps of the experiment: "The photon pairs are generated as in a rainbow, which allows us to distribute them unambiguously among the participants. Each participant then makes a measurement of the polarization of their 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 create cryptographic keys with each other and use them for tap-proof communication.
 
"A decisive advantage of this architecture is its flexibility," emphasizes Ursin. "We are thus able to integrate new communication partners into the quantum network with minimal intervention. This shows that quantum networks can become reality – for everyone. "
 
In the future, it will be possible to set up such comprehensive networks that all users, even over long distances, will be able to communicate with each other without the risk of eavesdropping. This is a prerequisite for the development of a quantum Internet. In further experiments, the researchers now aim to demonstrate the expandability of the new quantum network architecture. In the global race to develop a quantum Internet, at least one thing is clear: Alice and Bob will soon be able to network.