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Quantum comms succeed over metro-scale fibre networks

Experiments in China, Canada, get us a little closer to quantum repeaters

By Richard Chirgwin, 20 Sep 2016

Two groups both reported successfully teleporting state information across metro-scale fibre networks.

A Chinese team is claiming a teleportation over a 30 km fibre in Haifei (the entanglement travelled nearly 16 km), while the University of Calgary used that city's fibre network to carry entanglement 6.2 km (preprint here).

These are important steps forward for practical, long-distance quantum communications: as the Canadian study observes, it's a fundamental technology for quantum repeaters.

Each of the groups was following an “Alice-Bob-Charlie” quantum distribution model.

In the Canadian experiment:

  • Alice sends Photon A to Charlie; while Bob creates an entangled pair, B and C;
  • Bob sends B to Charlie and keeps C;
  • In what's called a Bell state measurement (BSM) Charlie measures A and B, in a way that prevents him knowing which photon he's measuring.

During the BSM, Alice's particle A teleports its quantum state onto Bob's particle C.

In the Chinese experiment, it was Charlie who created an entangled pair, one of which was sent to Bob.

Hang on, didn't the Chinese already do entanglement from a satellite?

Last month, China launched the Micius satellite. Part of that mission is a payload designed to test whether entanglement can be preserved and measured at around 500 km.

As it happens, that experiment demonstrates one of the limitations of quantum entanglement: the entanglement was carefully prepared on Earth before the satellite launch.

Back in 2012, a free-space quantum teleportation demonstration between La Palma and Tenerife measured entanglement over 143 km.

However, the Calgary team argue it's much more practical to build quantum communications networks that use infrastructure already in place – metropolitan fibres. ®

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