Advanced quantum network could be a prototype for the quantum internet

One of the most complex quantum networks built to date would allow 18 people to communicate securely using the power of quantum physics. The researchers behind the work say it offers a practical path to building a global quantum internet, but others are skeptical.

The long-promised quantum internet would allow quantum computers to communicate remotely by exchanging particles of light called photons, linked together through quantum entanglement. It would also make it possible to connect networks of quantum sensors or classical computers to send and receive unhackable communications. But connecting a quantum world is not as simple as laying cables, because ensuring that one node in the network can be entangled with another is a challenge.

Now, Xianfeng Chen of Jiao Tong University in Shanghai, China, and colleagues have shown how to connect two quantum networks. First, they built two networks, each with 10 nodes, all of which shared quantum entanglement – ​​effectively two tiny versions of a quantum Internet. They then sacrificed one node from each network to merge the two into a larger, fully entangled network in which every pair among the remaining 18 nodes could communicate.

Networking 18 classical computers would be a simple task, requiring only extremely cheap components, but in the quantum world it involves sharing individual photons between multiple users with such precise timing that it requires cutting-edge technology and expertise. Even communication between a pair of devices is complex, but allowing any pair among 18 users to communicate is unprecedented.

“Our approach provides a crucial capability for quantum communication across different networks and is advantageous for building a large-scale quantum Internet enabling communication among all users,” write the researchers, who did not respond to a request for comment, in a paper about their work.

This merging of networks, as the researchers describe it, requires a process called entanglement exchange. Photons can be quantumly entangled by performing a special observation called a Bell measurement. Simultaneously measuring the state of one photon from each of two pairs of entangled photons effectively connects the two farthest photons in the chain, but uses up the measured photons because any attempt to directly verify their state destroys the fragile quantum balance.

“This is not the first time that entanglement exchange has been demonstrated,” says Siddharth Joshi of the University of Bristol, UK. “What they did was they created a system to make the exchange between networks a little more convenient.”

Joshi explains that quantum communications research is currently divided between sending information between two devices over greater and greater distances, sometimes even with a satellite, and trying to create protocols and methods to reliably network many devices at short distances. This research falls into the latter camp. “Both of those things are very important,” he says.

But Robert Young of Lancaster University, UK, says that while the result is a phenomenal technical achievement that required skill and vast resources, he believes its cost and complexity make it unlikely to serve as a prototype for future large-scale quantum networks.

“It’s just very far from practical and very far from anything that could be implemented in the real world,” Young says. “The paper’s claim is that this is the future of how you could merge quantum networks, but there are so many challenges to get there that it’s frustrating.”

One of the problems lies in the need to use so-called quantum repeaters to send information over long distances. Photons are increasingly lost in fiber optic cables as distance increases, and because quantum information cannot be read and retransmitted (the measurement destroys the state of the photons), the signals cannot be amplified along the way. A working quantum repeater would allow signals to be transmitted over greater distances, but these devices have proven difficult to build.

“In practice, to build a quantum network, we know that we’re really going to need some form of quantum repeater,” says Young – something this network demonstration doesn’t address.