A Chinese research team has successfully achieved quantum entanglement between two quantum memories connected by 50 km long optical fiber, laying the foundation for building a quantum network based on quantum relay, according to a paper published in the Nature magazine on February 12.
The research was carried out in collaboration with the University of Science and Technology of China, the Jinan Institute of Quantum Technology, and the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences.
The team said that the quantum communication network development route widely used in academia is to achieve wide-area coverage through satellite-based free space channels, while using fiber-optic networks to achieve urban and inter-city ground coverage.
However, when photons are transmitted between nodes on an optical fiber, they are limited by the inherent attenuation of the optical fiber. At present, the farthest point-to-point ground safety communication distance is only on the order of one hundred kilometers.
"One way to extend the distance of quantum communication is to change point-to-point transmission to segmented transmission and use quantum relay technology for cascading." Xinhua News Agency cited Professor Pan Jianwei, a research leader, University of Science and Technology of China.
In other words, dividing the entire communication line into several segments, each with a small loss, and then connecting these segments through a quantum repeater, which makes it possible to build a full quantum network, he further explained.
However, limited by technical bottlenecks such as low entanglement of light and atoms, the farthest optical fiber quantum relay was only on the order of kilometers. To achieve the connection between long-distance quantum memories, the team has overcome several technical challenges.
For example, they independently developed a periodically polarized lithium niobate waveguide, which converted the light wavelength of the memory from near-infrared to the communication band through a non-linear difference frequency process. After 50 kilometers of optical fiber, it only attenuated to more than one percent. That's 16 orders of magnitude better than before.
In the experiment, the research team combined a number of new technologies to successfully achieve a two-node quantum entanglement between two quantum memories connected by a 50 km fiber. This distance is sufficient to connect the two cities.