Pan Jianwei, Bao Xiaohui, Zhang Qiang, and others from the University of Science and Technology of China used single photon interference for the first time to establish entanglement between independent storage nodes, and based on this, constructed the world's first urban three node quantum network based on entanglement. This work has increased the distance of real quantum entangled networks by three orders of magnitude to tens of kilometers from the previous tens of meters, laying a scientific and technological foundation for subsequent quantum network applications such as blind quantum computing, distributed quantum computing, and quantum enhanced long baseline interference. On May 15th, the relevant research results were published online in Nature.

The construction of quantum networks through remote transmission of quantum states is a fundamental element of large-scale quantum information processing. Based on quantum network, it is possible to realize wide area quantum key distribution, distributed quantum computing and quantum sensing, which forms the technical basis of the future "quantum internet". At present, quantum key networks based on single photon transmission have matured, and for further quantum network applications such as distributed quantum computing and distributed quantum sensing, it is necessary to use quantum relay technology to construct quantum entanglement between long-distance quantum memories. Based on this, various quantum information processing nodes are connected through wide area quantum teleportation.

In terms of quantum teleportation, Pan Jianwei's team has always been at the forefront of the world, achieving multi terminal, multi body, and multi degree of freedom quantum teleportation, laying a technical foundation for the transmission of quantum information in quantum networks. The team has been conducting long-term research on quantum storage and quantum relay. The team has taken the lead in implementing a stable quantum relay node with storage capabilities internationally; In order to improve key indicators such as storage lifetime, readout efficiency, and entanglement preparation probability, the team has developed multiple key technologies such as three-dimensional optical lattice cold atom quantum storage, ring cavity enhanced light atom interaction, and Rydberg blocking suppression high-order excitation. This not only achieves the best comprehensive performance of cold atom quantum storage, but also achieves deterministic light atom entanglement preparation.

On this basis, the team has made significant progress in the field of quantum storage networks in recent years. In 2019, the team achieved entanglement between three cold atom quantum memories in the laboratory through three-photon interference, becoming the first scalable quantum network prototype. In 2020, the team utilized quantum frequency conversion technology to convert the wavelength of emitted photons from quantum storage from 795 nanometers to 1342 nanometers, and combined it with single photon phase-locked technology to achieve dual node entanglement connected via a 50 kilometer fiber optic cable in the laboratory.

The main challenge in establishing entanglement between independent quantum memories separated at long distances is how to control the phase of a single photon. The entanglement scheme based on single photon interference has advantages in entanglement rate, but the experimental difficulty is very high. The subtle phase jitter caused by the control laser of quantum storage, frequency conversion pump laser, long fiber channel, etc. during the entanglement process can all lead to the final generation of entanglement decoherence. To solve this problem, this study designed and developed a very sophisticated phase control scheme: first, the laser linewidth is suppressed and controlled by stabilizing the frequency with an ultra stable cavity, second, the phase correlation between the read and write lasers is constructed through an optical phase-locked loop, and finally, the phase correlation between the two nodes is constructed through remote time-sharing phase comparison. By using the phase control technology mentioned above and utilizing quantum frequency conversion, the team has achieved entanglement between quantum memories located thousands of meters away. Based on this, the team constructed the world's first metropolitan three node quantum entanglement network. This network can establish entanglement between any two quantum memory nodes.



This work has increased the distance of real quantum entangled networks from tens of meters to tens of kilometers, laying the foundation for subsequent quantum network applications such as distributed quantum computing and distributed quantum sensing. This work is the first international metropolitan multi node quantum network experiment. The reviewers spoke highly of this work: "their achievement starts a new stage of quantum internet research", "paving the way for future large-scale quantum networks".



Nature also published the relevant experimental progress of the Lukin team at Harvard University in the United States. The team achieved long-distance entanglement of two nodes in the SiV color center system for the first time. Compared to the two, the achievements of the University of Science and Technology of China have a significant advantage in entanglement efficiency, which is more than two orders of magnitude higher than the work of Harvard University.



The research work was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, the Chinese Academy of Sciences, Anhui Province, etc.



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