Quantum hotline: information 'teleported' across cities
Two milestone studies demonstrate quantum teleportation across existing metropolitan fibre networks in China and Canada. Cathal O'Connell explains how it works.
While quantum teleportation – the remote transfer of quantum information from one location to another – has been achieved before, those previous works used purpose-built equipment secluded away in isolated labs.
These new studies show that quantum technologies can work with standard communication networks – an important step towards mainstream quantum cryptography – or even a quantum internet.
The studies are reported today in Nature Photonics.
Don’t be fooled by the teleportation you’ve seen in sci-fi – nobody has actually sent "beamed up" particles of matter anywhere. Quantum teleportation only concerns the transfer of information.
The cool thing is, the information gets from one place to another without passing through space in between.
Transmitting classical information across a city is a piece of cake. Alice can easily email a picture of her cat to her friend Bob – the picture is coded in strings of 1s and 0s and transferred from Alice’s computer, via the internet, until a copy winds up on Bob’s.
But what’s easy in the classical world is impossible in the quantum realm. Quantum uncertainty means you can never make a perfect measurement – or a perfect copy – of the quantum data. If Alice’s cat is coded in qubits – the quantum version of the bits of ordinary computing – she has no way of getting it to Bob in mint condition.
For a long time, physicists thought this "no copying" rule was insurmountable – something that would put a huge dampener on the usefulness of quantum information in general, as there would be no way to reliably shuttle it around. Then in 1993, physicists at IBM, along with a few colleagues, found a way around it: teleportation.
The IBM researchers realised that they could transport quantum data by exploiting quantum entanglement – the surreal connection between quantum particles which Albert Einstein dubbed "spooky action at a distance".
First Alice and Bob have to prearrange to share a pair of entangled particles. Think of this as a quantum hotline between them.
Alice then mixes her message with the entangled photons on her end. Although this step destroys the message on her end, it generates a “quantum key” of sorts which she can send to Bob over a regular internet connection.
Bob can then use the quantum key to “unlock” the information from his entangled particle.
the researchers synchronised the photons’ arrival time to within about a million billionth of a second
Since Bob – with his entangled particle – is the only one who can decode the message, quantum teleportation could be used to create an uncrackable quantum hotline.
There is a snag: the information at Alice’s end must be destroyed in the process. That’s why physicists call it teleportation. It’s as if the information disappears at one end and appears again on the other.
The crux of it is there is no "copy and paste" in the quantum world. Instead, there’s cut and paste (but physicists call it teleportation because – let's face it – it sounds much cooler).
The problem is entanglement is a delicate business. For instance, entangling photons involves sending them from different positions several kilometres apart and making sure they hit a detector at exactly the same time.
This is tricky enough in a lab, but the challenge is compounded with the noise and bustle of the real world jostling the line.
Now, as sometimes happens in science, two totally independent teams – one in China and the other in Canada – have cracked the same nut at the same time.
The teams each developed sophisticated systems to fine-tune the photons’ travel time, even as their optical fibre pathways lengthen or shrink with temperature.
The teams corrected this thermal stretch using tiny crystals that expand or contract when a voltage is passed through them.
This way the researchers synchronised the photons’ arrival time to within about a million billionth of a second. For a photon travelling at light speed – 300,000 kilometres per second – that means controlling its position to within a third of a millimetre.
The Hefei team succeed with about 50% of their transmissions – but at a sluggish rate of two teleported photons per hour. Using a simpler setup the Calgary team succeeded about 25% of the time, but achieved a relatively blistering 17 teleported photons per hour.
In an accompanying perspective piece, Frédéric Grosshans, a quantum physicist at the National Institute for Scientific Research in Paris, France, wrote that this performance would “strongly limit” practical applications if it could not be improved.
But on the whole he was optimistic: “For the longer term, the two papers demonstrate that the possibility of quantum networks that span a city are a realistic proposition, which is an exciting vision for the future.”