Quantum Teleportation Achieved Over 7km of Cable

Quantum teleportation was just achieved over more than 7 km of city fibre

Quantum teleportation just moved out of the lab and into the real world, with two independent teams of scientists successfully sending quantum information across several kilometres of optical fibre networks in Calgary, Canada, and Hefei, China.

The experiments show that not only is quantum teleportation very much real, it's also feasible technology that could one day help us build unhackable quantum communication systems that stretch across cities and maybe even continents.

Quantum teleportation relies on a strange phenomenon called quantum entanglement. Basically, quantum entanglement means that two particles are inextricably linked, so that measuring the state of one immediately affects the state of the other, no matter how far apart the two are - which led Einstein to call entanglement "spooky action at a distance".

Using that property, quantum teleportation allows the quantum state of one particle to be transferred to its partner, no matter the distance between the two, without anything physical passing between them.

That's not like the teleportation you see in sci-fi shows like Star Trek - only information can be sent via quantum teleportation, not people.

What it is, though, is a great way to create an unhackable, totally encrypted form of communication - just imagine receiving information that can only be interpreted once you know the state of your entangled particle.

In the latest experiments, both published in Nature Photonics (here and here), the teams had slightly different set-ups and results. But what they both had in common is the fact that they teleported their information across existing optical fibre networks - which is important if we ever want to build useable quantum communication systems.

In fact, quantum teleportation has been achieved over greater distances in the past - in 2012, researchers from Austria set a record by teleporting information across 143 km of space using lasers, but that technology isn't as useful for practical networks as optical fibre.

To understand the experiments, Anil Ananthaswamy over at New Scientist nicely breaks it down like this: picture three people involved - Alice, Bob, and Charlie. 

Alice and Bob want to share cryptographic keys, and to do that, they need Charlie's help. Alice sends a particle to Charlie, while Bob entangles two particles and sends just one of them to Charlie.

Charlie then measures the two particles he's received from each of them, so that they can no longer be differentiated - and that results in the quantum state of Alice's particle being transferred to Bob's entangled particle.

So basically, the quantum state of Alice's particle eventually ends up in Bob's particle, via a way station in the form of Charlie.

The Canadian experiment followed this same process, and was able to send quantum information over 6.2 km of Calgary's fibre optic network that's not regularly in use.

"The distance between Charlie and Bob, that's the distance that counts," lead researcher of the Canadian experiment, Wolfgang Tittel, from the University of Calgary in Alberta, told New Scientist. "We have shown that this works across a metropolitan fibre network, over 6.2 kilometres, as the crow flies."

The Chinese researchers were able to extend their teleportation further, over a 12.5 km area, but they had a slightly different set-up. It was Charlie in the middle who created the entangled particles and sent one to Bob, instead of the other way around.

This resulted in more accurate communication, and could work best for a quantum network where a central quantum computer (Charlie) communicates with lots of Alices and Bobs around a city. But the Calgary model could spread even greater distances, because Bob could work like a quantum repeater, sending the information further and further down the line.

The downside to both experiments was that they couldn't send very much information. The Calgary experiment was the fastest, managing to send just 17 photons a minute. 

And while many people assume that quantum teleportation would result in faster communication, in reality, decrypting the quantum state of the entangled particle requires a key, which needs to be sent via regular, slow communication - so quantum teleportation wouldn't actually be any faster than the internet we already have, just more secure.

But the fact that both teams were able to use existing telecommunications infrastructure to achieve such long-distance teleportation at all is a huge deal - and something that hasn't been done outside of the lab before.

It's going to take a lot more tweaking and investigation before it's something that we can use in our daily lives, but we're definitely getting closer.

From: https://www.sciencealert.com/quantum-teleportation-was-just-achieved-over-7-km-of-cable