One of the most sophisticated quantum networks constructed to date will enable 18 individuals to communicate securely through the principles of quantum physics. The researchers affirm that this represents a feasible step towards realizing a global quantum internet, although some experts express doubt.

The eagerly awaited quantum internet aims to allow quantum computers to communicate over distances by exchanging light particles, known as photons, that are interconnected through quantum entanglement. Additionally, it will facilitate the linkage of quantum sensor networks, enabling communications impervious to classical computer hacking. However, connecting different segments of the quantum realm is not as straightforward as laying down cables due to the challenges in ensuring seamless interactions between network nodes.

Recently, Chen Shenfeng from Shanghai Jiao Tong University in China demonstrated a method to interconnect two quantum networks. Initially, they established two networks containing 10 nodes each, both sharing quantum entanglement and functioning as smaller iterations of a quantum internet. They then combined one node from each network, resulting in a larger, fully integrated network that enables communication across all pairs of the 18 remaining nodes.

Networking 18 classical computers is a straightforward endeavor involving inexpensive components, but in the quantum sphere, where specific timing is crucial for sharing individual photons among several users, advanced technology and specialized knowledge are required. Even establishing communication between pairs is intricate, yet facilitating communication among any pair of 18 users is unprecedented.

“Our method provides essential capabilities for quantum communication across disparate networks and is pivotal for creating a large-scale quantum internet that enables interactions among all participants,” the researchers stated in their paper, which has not responded to inquiries for comments.

As the researchers clarify, this network integration hinges on a process termed entanglement swapping. Photons can be intertwined by conducting a specific observation known as the Bell measurement. By simultaneously measuring the status of one photon from each of two pairs of entangled photons, the most distant photons in the arrangement become linked. However, attempting to observe their states disrupts the delicate quantum balance and thus depletes the measured photon states.

“This isn’t the initial demonstration of entanglement exchange,” remarks Sidharth Joshi from the University of Bristol, UK. “What they have achieved is a framework that simplifies inter-network exchanges.”

Joshi notes that current quantum communication research is divided between extending the range of information transmission between two devices, occasionally utilizing satellites, and developing protocols and strategies for reliably networking numerous devices over shorter distances. This study pertains to the latter. “Both areas are critically important,” he asserts.

Conversely, Robert Young, a professor at Lancaster University in the UK, commented that while the results showcase a remarkable technical feat demanding expertise and extensive resources, he deems it improbable as a blueprint for future large-scale quantum networks, considering the expense and intricacy involved.

“This is far from practical and not something readily applicable in real-world scenarios,” Young states. “The paper’s claim is that this is the future of quantum network integration, but many formidable challenges remain to be addressed.”

One significant issue is the necessity for quantum repeaters to convey information across extensive distances. As distance increases, photons are frequently lost in fiber optic cables, and measurements can jeopardize the state of a photon, rendering the quantum information unreadable or untransmittable, thereby preventing signal amplification along its route. If quantum repeaters functioned effectively, they could transmit signals over longer distances, yet constructing such devices has been challenging.

“We understand that to build a viable quantum network, some method of quantum repeater is essential,” Young points out, emphasizing that this was absent in the current network demonstration.