The University of Science and Technology of China realizes long-distance entanglement between independent quantum memories

Academician Pan Jianwei of the University of Science and Technology of China and his colleagues Bao Xiaohui and Zhang Qiang combined long-life cold atom quantum storage technology with quantum frequency conversion technology, and used on-site optical fiber to establish entanglement between independent quantum storage nodes with a straight-line distance of 12.5 kilometers. The research results were published in the form of editors’ recommendations in the Physical Review Letters.


Schematic diagram of quantum storage node distribution courtesy of The University of Science and Technology of China

The basic unit of a quantum network is long-distance two-node entanglement. By using quantum storage technology to store photons, efficient entangled connections between different nodes will be possible. Constructing entanglement between memories and expanding node spacing has always been a research hotspot in the direction of quantum networks. In the implemented two-node entanglement experiment, the farthest straight-line distance is only 1.3 kilometers. In 2020, the team of Pan Jianwei of the University of Science and Technology of China made a breakthrough in this direction, expanding the distance of the two-node entangled optical fiber link to 50 kilometers. In this experiment, however, the two quantum memories were located in the same laboratory and did not achieve long-range separation.

To achieve long-distance separation of inter-memory entanglement, each quantum storage device needs to be able to operate independently. In this study, Node A is located in Hefei Innovation Industrial Park and Node B is located in the East District of The University of Science and Technology of China, and the two are connected by 20.5 kilometers of optical fiber. The team produced a long-lived light entangled with atoms at node A and sent the resulting single photons to node B after frequency conversion, and node B converted the received photons again and used another quantum memory for storage.

The experimental difficulties lie in the efficient transmission of single photons and long-life quantum storage. The team used laser-cooled rubidium atoms for quantum storage, with photon wavelengths of 795 nanometers, which are not suitable for transmission within long fibers. Using a periodic polarized lithium niobate waveguide developed by the Jinan Quantum Research Institute, the research team transferred the photon wavelength to 1342 nanometers, which greatly reduced the attenuation of photons in long optical fibers. Another difficulty lies in long-life quantum storage, which needs to exceed the photon transmission time.

To this end, the team designed a new light and atom entanglement generation scheme, while obtaining a long storage life, the resulting photon bits are encoded in time freedom, which is very suitable for frequency conversion and long-distance transmission.

On this basis, Pan Jianwei’s team successfully achieved long-distance entanglement between independent memories. This work lays the foundation for the subsequent construction of multi-node quantum network prototype systems, as well as quantum physical testing, and exploration of device-independent quantum key distribution. (Source: China Science Daily Wang Min)

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