Quantum memories entangled over 50-kilometer cable

Schematic of the remote entanglement generation between atomic ensembles. Credit: Nature (2020). DOI: 10.1038/s41586-020-1976-7

A team of Chinese researchers has succeeded in sending entangled quantum memories between buildings 20 kilometers apart and over a 50-kilometer coiled fiber cable.

The first experiment was based on the use of a small cloud of atoms placed in a desired quantum state. Reading and writing operations were done using photons. To engage the memory state, the researchers forced them into an optical cavity, allowing photons to interact with the atoms in the cloud. Once the memory state was set, the cloud emitted a photon to announce its readiness.

That photon was then polarized, allowing it to carry information regarding the state of the memory collective, which meant it could be used to entangle the memory. Preventing it from being lost during transmission required shifting its wavelength to one that is commonly used in fiber cable communications. The process proved to be approximately 30 percent efficient.

The second experiment involved creating just two quantum bits of memory from photons and sending them through 50 kilometers of coiled fiber.

The paper has been published in Nature. (Phys.org)

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