Entangled quantum memories, getting closer to the quantum internet

During the 1990s, important advances were made in the field of telecommunications, managing to extend the network to distances beyond cities and metropolitan areas, thus marking a before and after in global communication. In order to increase the scale of the system, repeaters were used, which improved the attenuated signals and allowed them to travel longer distances with the same intensity and fidelity characteristics. With the addition of satellites to the system, it has been possible to normalize the fact of being lost in the mountains in Europe and being able to talk with friends living on the other side of the world.

On the road to building the future quantum internet, quantum memories play that same role. Together with the qubit generating sources, they are the building blocks of this system, acting as repeaters of data operations using quantum superposition and entanglement as key ingredients. But, in order to handle a system like the quantum internet, it is first necessary to intertwine those memories over long distances, and maintain that entanglement in the most efficient way possible.

Dario Lago, Samuele Grandi, Alessandro Seri and Jelena Rakonjac, from the ICFO (Institute of Photonic Sciences) in the Barcelona town of Castelldefels, led by ICREA professor from ICFO Hugues de Riedmatten, have achieved, for the first time, a matter-matter intertwining between two quantum memories. This entanglement has been achieved between solid state memories, with multimode properties, remote (placed at a certain distance), and operating at the telecommunications wavelength, thus being a technology potentially adaptable on a larger scale. Put more simply; have managed to store, for a maximum of 25 microseconds, a single photon between two quantum memories separated from each other by 10 meters apart.

The researchers knew that the photon was in one of the two memories, but did not know which one. In fact, the photon would be in a state of quantum superposition in two memories at once, which, surprisingly, were 10 meters apart. The team learned that entanglement had been created by detecting a photon at the telecommunications wavelength, which was stored in quantum memories multiplexed, a technique that allows multiple messages to be sent simultaneously over a single communication channel. These two characteristics – achieving telecommunication wavelength interlacing and multiplexing – are key to being able to scale / extend the system over great distances, and have been achieved together for the first time.

A crystal doped with praseodymium (a chemical element of the rare earth group), used as quantum memory. (Photo: ICFO)

As Darío Lago, ICFO doctoral student and first author of the study, enthusiastically points out: “Until now, other groups had already achieved several of the milestones achieved in this experiment, such as entangling quantum memories or storing photons in quantum memories with an efficient and But the uniqueness of this experiment is that our techniques have achieved it jointly and efficiently, and that the system can be extended to great distances. “

Achieving this goal has required effort and time. The team prepared the experiment over the course of several months, using base crystals doped with praseodymium, a chemical element from the rare earth group, as quantum memories.

Two photon pair generator sources, correlated and individual, were also used. In each pair of photons, there was one called a “messenger”, with a length within the telecommunication range of 1436 nm; and the other, called “signal”, with a wavelength of 606 nm. The signal photons were sent to a quantum memory, made up of millions of randomly placed atoms inside a crystal, and stored there through a protocol called AFC – atomic frequency comb for short. In turn, the messenger photons were sent through an optical fiber to a device called a beam splitter, where information about their origin and trajectory was completely erased. Samuele Grandi, postdoctoral researcher and co-author of the study, comments: “We erased any type of characteristic that would tell us where the messenger photons came from, because we did not want to have any information about the signal photon or intuit in which quantum memory it was being stored.” By erasing these features, the signal photon could be stored in any of the quantum memories, which meant that there was entanglement between them.

To confirm and verify that an entanglement had indeed been achieved, the scientists saw a click on the monitor each time a messenger photon reached the detector. This entanglement was the signal photon in a state of superposition between the two quantum memories, being stored as an excitation shared by tens of millions of atoms for up to 25 microseconds.

As Sam and Darío mention, “The curious thing about the experiment is that it was not possible to know if the photon was stored in the quantum memory of laboratory 1 or laboratory 2, which were more than 10 meters apart. Although this is the main characteristic of our experiment, and therefore something we expected to happen, the results in the lab were still counterintuitive. And even more peculiar and mind-boggling to us, we were able to control it! “

Most previous studies have experimented with entanglement and quantum memories using messenger photons to find out whether entanglement between quantum memories had been successful or not. A messenger photon is like a homing pigeon, and scientists can know upon arrival that entanglement between quantum memories has been established. When this happens, the entanglement attempts are stopped and the entanglement is stored in memories before being analyzed.

In this experiment a messenger photon is used on the telecommunication frequency. Therefore, the entanglement that occurs could be established with a photon compatible with existing telecommunications networks. This represents a considerable feat, as it would allow long-distance entanglements to be created and for these quantum technologies to be easily integrated into existing classical telecommunications networks and infrastructures.

Multiplexing is the ability of a system to send several messages at the same time through a single transmission channel. In classical telecommunications, it is a tool that is frequently used to transmit data over the Internet. In quantum repeaters, this technique is a bit more complex. With standard quantum memories, one has to wait for the message announcing the entanglement to return to the memories before being able to try again to create a new entanglement. But through the AFC (Atomic Frequency Comb) protocol that enables this multiplexing approach, researchers can store entangled photons at many different times in quantum memory, without having to wait for the success signal to arrive before generating the next pair of entangled photons. This condition, called “temporal multiplexing” is a key characteristic that represents a significant increase in the operating time of the system, which leads to an increase in the final interleaving rate.

As Prof. ICREA from ICFO Hugues de Riedmatten comments: “We came up with this idea more than 10 years ago, and I am delighted to see that it has now been successful in the laboratory. The next steps are to take the experiment out of the laboratory, to try to link different nodes and distribute the entanglement over much greater distances, beyond what we have achieved now. In fact, we are in the middle of achieving the first 35-kilometer quantum link, which will be made between the city of Barcelona and the ICFO, in Castelldefels “.

It is clear that the future quantum network will bring many applications in the near future. Achieving this goal confirms that we are on the right path to develop these new technologies and begin to implement them in what will be a new form of communication, the Quantum Internet.

The new technical advance has been detailed by its authors in the academic journal Nature, under the title of “Telecom-heralded entanglement between multimode solid-state quantum memories”.

This study has received funding from the Quantum Internet Alliance (QIA) research project of the European initiative Quantum Flagship, the Gordon and Betty Moore foundation, as well as the Cellex Foundation, the Mir-Puig Foundation, the Generalitat de Catalunya, and the Spanish government, among other entities. (Source: ICFO)

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