Scientists have managed to get two quantum memories entangled over 50 kilometres (31 miles) of fibre optic cables, almost 40 times the previous record.
This achievement makes the idea of a super-fast, super-secure quantum internet a much more plausible one.
Quantum communication relies on quantum entanglement, or what Einstein called 'spooky action at a distance': where two particles become inextricably linked and reliant on each other, even if they're not in the same place.
Quantum memory is the quantum equivalent of classical computing memory – the ability to store quantum information and keep it for a later time – and if we're going to get to the stage where quantum computers are actually practical and useful, getting this fixed memory working is an important part.
"The main significance of this paper lies in extending the entangling distance in [optical] fibre between quantum memories to the city scale," team leader Jian-Wei Pan, of the University of Science and Technology of China, told the Australian Broadcasting Corporation.
As far as photon (light) particle entanglement goes, we've managed it over empty space and optic fibres at large distances in the past, but adding quantum memory makes the process much more complicated. The researchers suggest that a different type of approach might be better for this: atom-photon entanglement over successive nodes – where the atoms are the nodes and the photons transmit the messages.
In other words, photon entanglement with a twist, where atomic matter is added to the mix to produce extra efficiency, reliability, and stability.
With the right network of nodes, this could provide a better foundation to the quantum internet than pure quantum entanglement using photons alone.
In this experiment, the two storage units for quantum memory were rubidium atoms chilled down to a low energy state. When coupled with entangled photons, they each become part of an entangled system.
Unfortunately the greater the lengths a photon needs to go through to move between atoms, the greater the risk there is of that system being disturbed, which is why this new record is so impressive.
Key to the huge improvement in distance was a technique called cavity enhancement, which works to reduce photon coupling loss during entanglement.
In simple terms, this works by placing the quantum memory atoms into special rings this reduces random noise that could interfere and destroy the memory. The cavity has the added bonus of enhancing the retrieval of the quantum information.
The coupled atoms and photons produced by cavity enhancement make up the node. And the photons were then converted by the scientists to a frequency suitable for transmitting across telecommunications networks – in this case a telecommunications network the size of a city.
Pan's team has set a quantum entanglement record before, transmitting entangled photons between a satellite and Earth across a distance of 1,200 km (750 miles) in 2017. This satellite system works well in space, but in Earth's atmosphere with all the interference, fibre optic cables can reduce loss of signal.
In this experiment, the nodes of atoms were in the same lab, but the photons still had to travel across cables stretching more than 50 km. There are challenges in actually separating the atoms further, but the proof of concept is there.
"Despite enormous progress, at present the maximal physical separation achieved between two nodes is 1.3 kilometres [0.8 miles], and challenges for longer distances remain," explain the researchers in their published paper.
"Our experiment could be extended to nodes physically separated by similar distances, which would thus form a functional segment of the atomic quantum network, paving the way towards establishing atomic entanglement over many nodes and over much longer distances."
That's when things would get really interesting. While quantum memories might be the equivalent of computer memory in classical physics, the quantum version should be able to do much more – processing more information in a faster time, and solving conundrums beyond our current computers.
As far as communicating that data goes, quantum technology promises to improve transmission speeds and secure the data transfers using the laws of physics themselves – provided we can get it working in a reliable way over long distances.
"A quantum internet that connects remote quantum processors should enable a number of revolutionary applications such as distributed quantum computing," write the researchers in their published paper. "Its realisation will rely on entanglement of remote quantum memories over long distances."
The research has been published in Nature.