A team of researchers from the University of Science and Technology of China in Heifei gave yesterday an important step towards what specialists have called «quantum internet». His advances have just been presented in the journal Nature.

Under the direction of Jian-Wei Pan Y Xiao-Hui Bao, the researchers explain in their article how they have managed to intertwine two quantum memories located at 50 kilometers away the one of the other, a distance much greater than any of the obtained previously.

“A Quantum internet to connect remote quantum processors – says the article – should allow a series of revolutionary applications, such as distributed quantum computing. But its realization will depend on the entanglement of remote quantum memories for a long time and over long distances. Something that had not been achieved so far.

In fact, and despite the great progress of recent years, the maximum physical separation achieved between two nodes has been just 1.3 km. Wei Pan and Hui Bao have managed to extend this distance to 50 km. “Here – the scientists write in their article – we have demonstrated the entanglement of two atomic assemblies in a laboratory by means of photon transmission through optical fibers on a city scale. The sets of atoms function, in practice, as quantum memories that store quantum states.

For quantum communication to be effective, intertwined particles are needed. Quantum entanglement is a strange but tremendously useful property of matter at subatomic scale, so that when two particles are intertwined, the quantum state of any of them is immediately known to the other, regardless of the distance between them. Practically since this property is known, scientists have been trying to apply it to the field of telecommunications.

The problem is that maintaining entanglement over long distances and within, for example, a fiber optic cable, constitutes a tremendous challenge. Scientists have resorted to all kinds of stratagems (such as quantum repeaters) to ensure that entanglement does not break and is useful for sectors such as computer science (quantum computers) or telecommunications.

During the last two decades there have been great advances in this regard, achieving increasingly distant entanglements in which photons are successfully transmitted between the various nodes along fiber optic cables or by satelite. However, these advances had failed to end the serious problem of transmission loss. Due to these limitations, as has been said, the entanglement of two quantum memories (which is the quantum equivalent of ordinary computer memory) has not been able to be maintained beyond 1.3 km.

Now, Jian-Wei Pan, Xiao-Hui Bao and their colleagues have managed to multiply that distance by almost fifty. And they have done so by exploiting a quantum effect called «cavity improvement», Which significantly reduces the loss of photon coupling during transmission. In this way, the researchers managed to create intertwined atoms and photons, then convert them to a frequency suitable for telecommunications.

The success is unquestionable, because thanks to this system the scientists were able to stably interconnect nodes connected by up to 50 km of optical cables. The results show that, in comparison to the entanglement of photons, intertwining entire atoms with photons gives many better results in transmission.

In the words of the researchers themselves, «we use cavity enhancement to efficiently create atom-photon entanglement, and we use quantum frequency conversion to change the atomic wavelength to telecommunications wavelengths. We observe entanglements in more than 22 kilometers of fibers deployed in a field through interference of two photons, and entangled in more than 50 kilometers of fibers wound by interference of a single photon ».

“Our experiment – scientists conclude – could be extended to nodes physically separated by similar distances, which would form a functional segment of an atomic quantum network, paving the way to atomic entanglement in multiple nodes and in much longer distances.” Or what is the same, towards a quantum Internet.

José Manuel Nieves Updated:02/13/2020 05: 47h

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