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.
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.
The current work is scalable with Quantum repeaters. This could allow communication across arbitrary distances
If a photon interacts with a member of an entangled photon pair via a so-called Bell-state measurement (BSM), its state is teleported over principally arbitrary distances onto the second member of the pair. Starting in 1997, this puzzling prediction of quantum mechanics has been demonstrated many times; however, with one very recent exception, only the photon that received the teleported state, if any, travelled far while the photons partaking in the BSM were always measured closely to where they were created. Here, using the Calgary fibre network, we report quantum teleportation from a telecommunication-wavelength photon, interacting with another telecommunication photon after both have travelled over several kilometres in bee-line, onto a photon at 795~nm wavelength. This improves the distance over which teleportation takes place from 818~m to 6.2~km. Our demonstration establishes an important requirement for quantum repeater-based communications and constitutes a milestone on the path to a global quantum Internet.
Arxiv - Quantum teleportation across a metropolitan fibre network
Nature Photonics - Quantum teleportation across a metropolitan fibre network
Nature Photonic - Quantum teleportation with independent sources and prior entanglement distribution over a network
SOURCES- New Scientist, arxiv, Nature Photonics