Scientists entangled two different quantum nodes 12.5 km apart from each other

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Nergis Firtina
Quantum computer.
Quantum computer.


  • Researchers from China have recently demonstrated quantum entanglement between two memory devices.
  • Two different quantum nodes in an urban environment were placed 41010 feet (12.5 km) apart.
  • This research shows that it can pave the way for the establishment of a quantum internet in cities.

Quantum computers have the power to work and process information more powerfully than the classical computers used today. Lately, some researchers have also been finding out to set up quantum internet, which allows quantum devices to exchange information as classical computing devices do.

Researchers at the University of Science and Technology of China and Jinan Institute of Quantum Technology have recently demonstrated quantum entanglement between two memory devices located 41010 feet (12.5 km) apart from each other within an urban environment. The research has been published in Physical Review Letters.

"In 2020, we published a paper in which we demonstrate the entanglement of two quantum memories via a fiber link of 50 km (164041 feet)," said Xiao-Hui Bao, one of the researchers who carried out the study.

"In that experiment, both two memories we used were located within one lab and thus not fully independent. The next step in our research was to make the two memories fully independent while placing a long distance between them."

Two different nodes were introduced to different locations in the town

In experiments by Bao and colleagues, two different quantum nodes in an urban environment were placed 41010 feet (12.5 km) apart. In the first node, dubbed node A, they entangled their first quantum memory with a single photon. This single photon was then sent to node B and stored within the second quantum memory.

"In this way, we entangle the two remote quantum memories," Bao explained. "Since the photon emitted from our memory is near infrared (795 nm), being not suitable for low-loss transmission in fiber, we make use of the quantum frequency conversion technique to shift the photon's wavelength to 1342 nm instead, which improves the overall transmission efficiency significantly."

"The main achievement of our recent work is that we realized the longest distance of entanglement distribution with quantum memories," Bao said. "Such entanglement is the fundamental resource to build a quantum network and quantum repeaters."

This latest work by Bao and colleagues, the research focusing on quantum technology and the establishment of the quantum internet, sheds light on the research that will follow in this area.

"In the current experiment, the remote entanglement generated is not heralded yet, limiting its further applications," Bao added. "In the near future, we plan to implement a heralded version, meanwhile we plan to extend a number of nodes as well."

Study abstract

A quantum internet that connects remote quantum processors should enable a number of revolutionary applications such as distributed quantum computing. Its realization will rely on the entanglement of remote quantum memories over long distances. Despite enormous progress at present the maximal physical separation achieved between two nodes is 1.3 kilometres (4265 feet), and challenges for longer distances remain. Here we demonstrate entanglement of two atomic ensembles in one laboratory via photon transmission through city-scale optical fibres. The atomic ensembles function as quantum memories that store quantum states. We use cavity enhancement to efficiently create atom–photon entanglement and we use quantum frequency conversion to shift the atomic wavelength to telecommunications wavelengths. We realize entanglement over 22 kilometres (72178 feet) of field-deployed fibres via two-photon interference and entanglement over 50 kilometres (164041 feet) of coiled fibres via single-photon interference. Our experiment could be extended to nodes physically separated by similar distances, which would thus form a functional segment of the atomic quantum network, paving the way towards establishing atomic entanglement over many nodes and over much longer distances.

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