Scientists are edging closer to making a super-secure, super-fast quantum internet possible: they've now been able to 'teleport' high-fidelity quantum information over a total distance of 44 kilometres (27 miles).
Both data fidelity and transfer distance are crucial when it comes to building a real, working quantum internet, and making progress in either of these areas is cause for celebration for those building our next-generation communications network.
In this case the team achieved a greater than 90 percent fidelity (data accuracy) level with its quantum information, as well as sending it across extensive fibre optic networks similar to those that form the backbone of our existing internet.
"We're thrilled by these results," says physicist Panagiotis Spentzouris, from the Fermilab particle physics and accelerator laboratory based at the California Institute of Technology (Caltech).
"This is a key achievement on the way to building a technology that will redefine how we conduct global communication."
Quantum internet technology uses qubits; unmeasured particles that remain suspended in a mix of possible states like spinning dice yet to settle.
Qubits that are introduced to one another have their identities 'entangled' in ways that become obvious once they're finally measured. Imagine these entangled qubits as a pair of dice - while each can land on any number, they are both guaranteed to add to seven no matter how far apart they are. Data in one location instantly reflects data in another.
By clever arrangement of entangling three qubits, it's possible to force the state of one particle to adopt the 'dice roll' of another via their mutually entangled partner. In quantum land, this is as good as turning one particle into another, teleporting its identity across a distance in a blink.
The unstable, delicate nature of quantum information makes it tricky to beam entangled photons over long distances without interference, however. Longer optical fibres simply mean more opportunity for noise to interfere with the entangled states.
In total, the lengths of fibre used to channel each cubit added to 44 kilometres, setting a new limit to how far we can send entangled qubits and still successfully use them to teleport quantum information.
It's never before been demonstrated to work over such a long distance with such accuracy, and it brings a city-sized quantum network closer to reality – even though there are still years of work ahead to make that possible.
"With this demonstration we're beginning to lay the foundation for the construction of a Chicago-area metropolitan quantum network," says Spentzouris.
Quantum entanglement and data teleportation is a complex science, and not even the experts fully understand how it might ultimately be used in a quantum network. Each proof of concept like this that we get puts us a little closer to making such a network happen though.
As well as promising huge boosts in speed and computational power, a quantum internet would be ultra-secure – any hacking attempt would be as good as destroying the lock being picked. For now at least, scientists think quantum internet networks will act as specialist extensions to the classical internet, rather than a complete replacement.
Researchers are tackling quantum internet problems from all different angles, which is why you'll see a variety of distances mentioned in studies – they're not all measuring the same technology, using the same equipment, to test the same standards.
What makes this study special is the accuracy and the distance of the quantum entanglement teleportation, as well as the 'off the shelf' equipment used – it should theoretically be relatively easy to scale up this technology using the hardware we're already got in place.
"We are very proud to have achieved this milestone on sustainable, high-performing and scalable quantum teleportation systems," says physicist Maria Spiropulu, from Caltech.
"The results will be further improved with system upgrades we are expecting to complete by the second quarter of 2021."
The research has been published in PRX Quantum.