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A new world record for quantum teleportation has been set, bringing quantum communication networks that can stretch between cities a step closer. Two independent teams have transferred quantum information over several kilometres of fibre optic networks.

Being able to establish teleportation over long distances is a crucial step towards exchanging quantum cryptographic keys needed for encoding data sent over the fibres.

Quantum teleportation is a phenomenon in which the quantum states of one particle can be transferred to another, distant particle without anything physical traveling between them. It relies on a property called entanglement, in which measuring the state of one particle immediately affects the state of its entangled partner, regardless of the distance between them.

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Conceptually, one way of doing teleportation involves three participants: say, Alice, Bob and Charlie. In order for Alice and Bob to exchange cryptographic keys, they have to first establish the capacity for teleportation, with Charlie’s help.

First Alice sends a particle (A) to Charlie. Bob, meanwhile, creates a pair of entangled particles (B & C), sends B to Charlie and holds on to C. Charlie receives both A and B, and measures the particles in such a way that it’s impossible to tell which particle was sent by Alice and which by Bob. This so-called Bell state measurement results in the quantum state of particle A being transferred to particle C, which is with Bob.

Stretching further

In the first teleportation experiments performed in 1997, Alice, Bob and Charlie were on the same optical bench in the same laboratory. The distances involved were few tens of centimetres. If teleportation distance is defined as the distance between Charlie and Bob, then until recently the record was a mere 800 metres, because doing a Bell state measurement was difficult with photons that had travelled too far.

The record for sheer distance between Alice and Bob was set in 2012, when a group led by Anton Zeilinger at the University of Vienna achieved teleportation over 143 kilometres of free space between two of the Canary Islands. But there’s no obvious way to translate that feat into a practical quantum network that would work within a city, where free space is hard to come by and other interference would destroy delicate quantum states.

Now, Wolfgang Tittel at the University of Calgary in Alberta, Canada, and colleagues have upped the ante. They extended the distance between Charlie and Bob and teleported quantum states using part of Calgary’s fibre optic network that isn’t being used for regular communications.

“The distance between Charlie and Bob, that’s the distance that counts,” says Tittel. “We have shown that this works across a metropolitan fibre network, over 6.2 kilometres, as the crow flies.”

Jian-Wei Pan at the University of Science and Technology of China and colleagues achieved a comparable separation between Charlie and Bob when they teleported quantum states using the city of Hefei’s fibre optic network. Their setup was slightly different, though: it was Charlie in the middle who created the entangled pair of particles and sent one to Bob, instead of the other way around.

Repeated feat

Pan’s configuration could prove useful for building a quantum network within a city, where many sets of Alice-Charlie-Bob links each communicate with a central quantum computer. But Tittel’s team argues that their setup could enable quantum communication networks that stretch between cities.

That’s because it would allow for the creation of quantum repeaters, to propel the signal further along the network. Say Alice and Bob are 100 kilometres apart, with Bob to the right of Alice. Both create a pair of entangled particles each, keep one and send the other to Charlie, who is midway between them. Charlie performs the Bell state measurement, entangling the particles still with Alice and Bob.

Now say Bob repeats the process with Daisy, who is 100 kilometres to his right (with another Charlie between them). At this stage, Bob has two particles, one entangled with Alice’s and the other with Daisy’s. If Bob now does a Bell State measurement on his two particles, he effectively entangles Alice’s particle with Daisy’s — stretching teleportation a full 200 kilometres.

“You can scale the whole thing up and can go, in theory, to arbitrarily long distances,” says Tittel.

“The two experiments can be seen as milestones on the path to a long-term goal, namely to build a fibre-based quantum internet connecting large cities,” says Johannes Kofler at the Max Planck Institute of Quantum Optics in Munich.

Journal reference: Nature Photonics, DOI: 10.1038/nphoton.2016.180

Journal reference: Nature Photonics, DOI: 10.1038/nphoton.2016.179