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Dec 20 2018

Researchers Expand Number of Possible Participants in Quantum Network

Tap-proof internet communication is an urgent need worldwide. One possible way to enable it is to further develop quantum cryptography. A team of scientists from the University of Vienna has now overcome a decisive hurdle.

Providing senders and recipients with a shielded line for communication protected by quantum cryptography is not exactly a new thing; however, so far most efforts suffered from the fact that protocols and implementation mechanisms for exchanging the required quantum keys were limited to two parties. An international group of researchers led by Austrian experimental physicist Rupert Ursin, who's currently a deputy director at the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences, has now developed a 'passive' distribution method that will exceed existing limitations and could ultimately give everyone access to quantum networks and protected communications.

Like all quantum computing concepts and solutions, quantum cryptography is based on entanglement – a phenomenon first described by quantum physicists who found that subatomic particles can take on a common state and remain connected even if they're subsequently separated over long distances. The properties of the two individual particles are indeterminate for as long as no one interferes. But entanglement ends as soon as a property for one of the two particles is measured, because the measurement as such triggers the other particle to immediately assume a corresponding property. As a result, potential adversaries cannot attack the key exchange mechanism without immediately revealing their presence.

At this point, one might say so far, so cool – but how does this fix the basic issue that only two parties can connect in a quantum network? Ursin and his team now claim to have found a method to "scale the advantages of quantum key distribution protocols to more than two distant users." They present "a fully connected quantum network architecture in which a single entangled photon source distributes quantum states to many users while minimizing the resources required for each (...) without sacrificing security or functionality relative to two-party communication schemes." Technically, they're using a single source of bipartite polarization entanglement and multiplex it into 12 wavelength channels. Six states are then distributed between four participants using only one fibre and one polarization analysis module per user. As a result, no "adaptations of the entanglement source" are necessary to add more users, thus ultimately allowing for the long-awaited flexibility.

The Austrian findings were published in an article in Nature Vol. 564 ("An entanglement-based wavelength-multiplexed quantum communication network"). The piece (PDF) is available for download from the Nature web page; the magazine charges $8.99 for 48 hour rent, cloud access costs $22, and full access to the 9-pager sets you back $32.


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