# Pan Jianwei team achieves quantum memory entanglement 50km away

The China University of Science and Technology recently demonstrated the entanglement of two quantum memorys 50 kilometers apart, providing ideas for solving the key technical problems of large-scale quantum Internet. Previously, quantum entanglement between two fixed nodes could never exceed a distance of 1.3 km. The paper was published in the early hours of February 13 th in the british journal Nature, the world’s leading academic journal. Professor Pan Jianwei, Bao Xiaohui and Zhang Qiang are the authors of the article.

(Original title: Pan Jianwei team achieves quantum memory entanglement 50 kilometers away, 37 times over record)

Journalist Yu Hanqi

Quantum is a fundamental unit of physics that cannot be divided, for example, photons are quantum, and there is no term for “half photons”. In this microcosm, scientists have discovered many wonderful properties based on the development of concepts such as quantum communication and quantum computing.

Quantum entanglement is one of them, which Einstein called “a ghostly long-range effect”. The two quantum states in a entangled state are related no matter how far apart, with one changing the quantum state (such as being measured) and the other changing instantaneously, as if it were telepathic.

Although this is the case in theory, there are still many technical challenges to build a large-scale quantum internet to ensure that two distant nodes remain so delicately entangled.

Since the 1970s, physicists have experimented with long-distance quantum entanglement distribution, which is to distribute entangled photons in two places.

As early as 2005, Pan Jianwei’s team achieved 13 kilometers of quantum entanglement distribution in Hefei’s Otsuka Mountain. In 2012, the team conducted the first quantum entanglement distribution experiment in Qinghai Lake, more than 102 kilometers. In 2017, using the world’s first quantum communications experimental satellite, the Moko, they set a world record for quantum entanglement distribution distance, reaching 1,200 kilometers.

However, such a star-earth quantum entanglement distribution causes a large transmission loss, in practical application s If photons are to be transmitted over long distances, there will also be severe losses, limiting the success rate of distribution.

One solution, the paper notes, is to prepare quantum memory (a substance that stores quantum states) and a photon on two long-distance nodes, and then transfer the two photons to a common intermediate node. After proper measurement of the two photons, two quantum memorys on the primary node can be projected into a remote entanglement state.

Long-distance entanglement of two clusters of atoms through intermediate nodes

Previously, international scientists have used atomic clusters, atoms, diamond in the nitrogen space color center, ion trap and other systems as quantum memory operation, but the best results are only 1.3 kilometers.

The paper points out that there are three main challenges to expand the entanglement distance to the intercity scale. The first is to obtain “bright” (i.e. effective) material-photon entanglement, the second is to reduce transmission losses, and the third is to achieve long-distance fiber in the stability of high visible interference.

To address these challenges, the team used a quantum effect called cavity enhancement to prepare bright clusters of atoms and photon entanglement. They set up two such nodes on the campus of the Chinese University of Science and Technology, and then transferred two messenger photons to the intermediate node, Hefei Software Park, through two parallel 11 km-long fiber optics.

To reduce photon loss during transmission, the team used quantum frequency conversion technology to convert photons from near-infrared frequencies to frequencies suitable for telecommunications transmission. Finally, they realized the entanglement of two quantum nodes under the dual photon interference mechanism, which is equivalent to crossing 22 kilometers.

Next, the Chinese University of Science and Technology team went one step further, under the single photon interference mechanism, so that two nodes connected by 50 kilometers of fiber optic to achieve entanglement, reached the intercity scale.

The paper optimistically concluded that by connecting more distance-like nodes, the experiment could expand into a functional unit of a quantum network, paving the way for a large-scale quantum Internet.