Pan Jianwei's team realizes long-distance entanglement between independent quantum memories for the first time

Academician Pan Jianwei.

The reporter learned from the University of Science and Technology of China on the 7th that the school's Pan Jianwei and his colleagues Bao Xiaohui, Zhang Qiang, etc., combined long-life cold atom quantum storage technology with quantum frequency conversion technology, using on-site optical fibers at a distance of 12.5 kilometers. Entanglement is established between quantum storage nodes. Relevant research results were recently published in the "Physical Review Letters" in the form of editor's recommendation.
The fundamental unit of quantum networks is long-distance two-node entanglement. Storing photons using quantum memory technology will enable efficient entangled connections between different nodes. Constructing inter-memory entanglement and expanding the node spacing has always been a research hotspot in the direction of quantum networks. In the realized two-node entanglement experiment, the farthest straight-line distance is only 1.3 kilometers. In 2020, the team of Pan Jianwei of China University of Science and Technology made a breakthrough in this direction, extending the distance of the two-node entangled optical fiber link to 50 kilometers. However, in this experiment, the two quantum memories were located in the same laboratory and did not achieve long-range separation.
To achieve long-range separated inter-memory entanglement, each quantum memory device needs to be able to be manipulated independently. In this study, node A is located in Hefei Innovation Industrial Park, and node B is located in the East District of China University of Science and Technology, and the two are connected by 20.5 kilometers of optical fiber. The team generated a long-lived entanglement of light and atoms at node A, and sent the generated single photon to node B after frequency conversion. Node B converted the received photon again and stored it in another quantum memory.
The experimental difficulty lies in the efficient transmission of single photons and long-lived quantum storage. The team used a periodically polarized lithium niobate waveguide developed by Jinan Quantum Research Institute to transfer the photon wavelength to 1342 nanometers, which greatly reduced the attenuation of photons in long fibers. Another difficulty lies in long-life quantum storage, where the storage life needs to exceed the photon transmission time. To this end, the team designed a new generation scheme for entanglement of light and atoms. While obtaining a long storage life, the generated photon bits are encoded in the degree of freedom in time, which is very suitable for frequency conversion and long-distance transmission.
(Original title "My team realizes long-distance entanglement between independent quantum memories for the first time)")