HEFEI -- A research team from the University of Science and Technology of China (USTC) has achieved significant advances in the field of scalable quantum networks, bringing this transformative technology closer to real-world application. Their landmark findings have been published in both Nature and Science.

A central goal of quantum information science is the creation of highly efficient and ultra-secure quantum networks, which require the long-distance distribution of quantum entanglement -- a phenomenon involving a unique connection between particles. Such entanglement is essential to enable quantum-secure communication and interconnect future quantum computers. A major obstacle, however, has been signal loss in optical fibers, where transmission efficiency drops drastically with distance, making large-scale networks impractical.

To address this issue, the team focused on a concept known as a "quantum repeater," which breaks a long communication link into shorter segments, establishes entanglement within each, and then connects them. The key challenge has been that quantum entanglement is typically too short-lived to outlast the time needed to link segments, preventing the repeater from functioning effectively.

The USTC team overcame this fundamental limitation by developing a long-lived trapped-ion quantum memory, a highly efficient ion-photon interface, and a high-fidelity experimental protocol. Together, these innovations enabled quantum entanglement that persists significantly longer than the time required to establish inter-segment connections.

According to USTC, this is the world's first demonstration of a scalable building block for a quantum repeater -- a critical step toward long-distance quantum networks.

In a related breakthrough, the team used similar technology to generate high-fidelity entanglement between two distant rubidium atoms. Leveraging this, they demonstrated device-independent quantum key distribution (DI-QKD) over city-scale fiber networks for the first time.

DI-QKD is regarded as the gold standard for secure communication, as its security is guaranteed by the laws of quantum physics, independent of any potential device flaws.

The team successfully implemented DI-QKD over 11 kilometers of fiber, extending the attainable distance approximately 3,000 times beyond previous results. They also confirmed the feasibility of generating secure keys over a distance of 100 kilometers, surpassing the prior international record by more than two orders of magnitude.

The researchers have hailed these outcomes as pivotal milestones for China in the field of quantum communication and networking, signaling that fiber-based quantum networks are advancing from a theoretical concept toward practical implementation.