Advanced quantum network could be a prototype for the quantum internet

One of the most complex quantum networks created to date, 18 people will be able to communicate securely using the power of quantum physics. The researchers behind the work say it offers a practical path to creating a global quantum internet, but others are skeptical.

Long Promised as much as the Internet will allow quantum computers communicate over a distance by exchanging particles of light called photons that are bound together quantum entanglement. This would also allow networks quantum sensors for communication or classical computers for sending and receiving messages that cannot be hacked. But connecting the quantum world together is not as simple as laying cables, because ensuring that one network node can be entangled with another is a challenge.

Now, Xianfeng Chen from Shanghai Jiao Tong University (China) and his colleagues showed how to link two quantum networks together. First, they built two networks, each with 10 nodes, that share quantum entanglement—essentially two tiny versions of the quantum internet. They then sacrificed one node from each network to combine them into one large, fully entangled network in which every pair of the 18 remaining nodes could interact.

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

“Our approach offers critical capabilities for quantum communication between different networks and is beneficial for creating a large-scale quantum Internet that enables communication between 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 made quantum entangled by making a special observation called Bell measurement. Simultaneously measuring the state of one photon from each of two pairs of entangled photons effectively links the two most distant photons in the chain, but wastes the measured photons since any attempt to directly test their state destroys the fragile quantum balance.

“This is not the first time entanglement exchange has been demonstrated,” says Siddharth Joshi at the University of Bristol, UK. “They created a design that allows you to switch between networks in a more convenient way.”

Joshi says research into quantum communication is currently divided between sending information between two devices over a network. increasingly long distancessometimes even with one on a satellite, and trying to create protocols and methods for reliably networking many devices over short distances. This study falls into the second camp. “Both of these factors are very important,” he says.

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

“It's so far from practicality and from anything that could be implemented in the real world,” Young says. “The paper claims that this is the future of how quantum networks can be connected, but to achieve this goal there are so many problems that need to be solved that it is disappointing.”

One problem is the need for so-called quantum repeaters to transmit information over long distances. Photons are increasingly lost in fiber optic cables as distance increases, and since quantum information cannot be read and retransmitted (since measurements destroy the photons' state), signals cannot be amplified along their path. A working quantum repeater would allow signals to be transmitted over long distances, but building these devices has proven difficult.

“In practice, when we build a quantum network, we know that we will actually need some form of quantum repeater,” Young says, something that this network demonstration does not address.