Quantum Memory Milestone Boosts Quantum Internet Future

Record efficiency in storing and retrieving quantum entanglement paves the way for connecting quantum internet nodes

2 min read
Record efficiency in storing and retrieving quantum entanglement paves the way for connecting quantum internet nodes.
Illustration: iStockphoto

An array of cesium atoms just 2.5 centimeters long has demonstrated a record level of storage-and-retrieval efficiency for quantum memory. It’s a pivotal step on the road to eventually building large-scale quantum communication networks spanning entire continents.

A quantum internet could connect far-flung communication nodes through entanglement—a phenomenon that enables quantum-mechanically connected particles to experience related changes to their respective energy states regardless of the distance between them. But the time it takes for these systems to distribute their entanglement depends, naturally, on their efficiency at storing and retrieving entanglement. 

“We learned in this work [how] to achieve this 85-90 percent [efficiency] benchmark,” says Julien Laurat, a professor and leader of the quantum networks team at the Kastler Brossel Laboratory at Sorbonne University in France. “This is the best in any physical platform.”

Previous work only achieved at most 25 percent efficiency, as Laurat and his colleagues describe in a paper published in a recent issue of the journalOpticaTheir demonstration is part of a broader effort by the Quantum Internet Alliance—a coalition of university research groups from eight European countries—to develop technologies for a European network that supports quantum communication and distributed quantum computing. 

Boosting storage-and-retrieval efficiency from 25 percent to 90 percent makes a big difference in the speed and size of the quantum network such memory devices could support. For instance, Laurat and colleagues note, increasing efficiency from 60 percent to 90 percent would accelerate quantum memory speeds by two orders of magnitude over a distance of 600 km. 

That said, much work remains before quantum networks can get to be hundreds of kilometers across.

The experiment involved two quantum memory devices based on ensembles of laser-cooled cesium atoms. Researchers demonstrated how cesium atoms can store and retrieve single-photon entanglement from entangled light beams. Entangled photons could connect the various quantum nodes in a future quantum internet.

“In future large-scale quantum networks, multiple such entangled links will need to be generated at the same time in order to be able to later connect them into an end-to-end long-distance link,” says Filip Rozpedek, a postdoctoral associate researching quantum communication at the University of Chicago, who was not involved in the study. “Storage in quantum memories is needed in order to be able to maintain a given link while waiting for successful generation of the other ones.”

This “quantum repeater” protocol of connecting smaller segments into a larger quantum network would not be practical without reliable storage-and-retrieval efficiency. And, Rozpedek says, that efficiency “opens the possibility of experimentally investigating such practical quantum repeaters.” 

The next big step, Rozpedek explains, would involve demonstrating high transfer efficiency between quantum memory devices separated by a larger distance and with longer storage time. Future success in building a quantum internet would also likely require use of a “multiplexing” technique that allows for parallel attempts to establish entangled links between the quantum nodes.

Laurat and his team already have their sights set on tackling some of those next milestones. If all goes well, they envision a pan-European quantum network becoming possible within the next five to 10 years. “The next few years should see demonstrations over a few tens of kilometers,” Laurat says.

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Metamaterials Could Solve One of 6G’s Big Problems

There’s plenty of bandwidth available if we use reconfigurable intelligent surfaces

12 min read
An illustration depicting cellphone users at street level in a city, with wireless signals reaching them via reflecting surfaces.

Ground level in a typical urban canyon, shielded by tall buildings, will be inaccessible to some 6G frequencies. Deft placement of reconfigurable intelligent surfaces [yellow] will enable the signals to pervade these areas.

Chris Philpot

For all the tumultuous revolution in wireless technology over the past several decades, there have been a couple of constants. One is the overcrowding of radio bands, and the other is the move to escape that congestion by exploiting higher and higher frequencies. And today, as engineers roll out 5G and plan for 6G wireless, they find themselves at a crossroads: After years of designing superefficient transmitters and receivers, and of compensating for the signal losses at the end points of a radio channel, they’re beginning to realize that they are approaching the practical limits of transmitter and receiver efficiency. From now on, to get high performance as we go to higher frequencies, we will need to engineer the wireless channel itself. But how can we possibly engineer and control a wireless environment, which is determined by a host of factors, many of them random and therefore unpredictable?

Perhaps the most promising solution, right now, is to use reconfigurable intelligent surfaces. These are planar structures typically ranging in size from about 100 square centimeters to about 5 square meters or more, depending on the frequency and other factors. These surfaces use advanced substances called metamaterials to reflect and refract electromagnetic waves. Thin two-dimensional metamaterials, known as metasurfaces, can be designed to sense the local electromagnetic environment and tune the wave’s key properties, such as its amplitude, phase, and polarization, as the wave is reflected or refracted by the surface. So as the waves fall on such a surface, it can alter the incident waves’ direction so as to strengthen the channel. In fact, these metasurfaces can be programmed to make these changes dynamically, reconfiguring the signal in real time in response to changes in the wireless channel. Think of reconfigurable intelligent surfaces as the next evolution of the repeater concept.

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