Quantum entanglement. Telecom-heralded entanglement between multimode solid-state quantum memories

Until now, the entanglement of quantum memories and the storage of photons inside had been achieved separately, but researchers at the Institute of Photonic Sciences have achieved everything at the same time: keeping a photon in a quantum superposition state for 25 microseconds in two separate devices 10m away. The technique is compatible with the current telecommunications network and will help the development of quantum repeaters.

Our telecommunications systems require the use of repeaters, both on the ground and through satellites, so that the signals are maintained and can travel long distances. In developments towards the future quantum internet, quantum memories play the same role. Together with the generating sources of the qubits or quantum bits, they will be the other basic component of the system.

These new memories act as repeaters of data operations using two characteristics of the quantum world: superposition (the possibility that a particle is in several states at the same time, like Schrödinger’s dead and alive cat) and entanglement (correlation that is between two distant particles, in such a way that the interaction with one affects the other).

But to get to the quantum internet, it is first necessary to entangle quantum memories over long distances and maintain the entanglement as efficiently as possible.

In this context, researchers from the Institute of Photonic Sciences (ICFO) in Castelldefels (Barcelona) have taken a big step, according to the study published on the cover of the journal Nature.

The authors have managed to store, for a maximum of 25 microseconds, a single photon between two quantum memories separated from each other by a distance of 10 m. They knew that this particle was in one of the two memories, but not in which one, a common situation in the counterintuitive quantum world.

The authors have managed to store, for a maximum of 25 microseconds, a single photon between two quantum memories separated from each other by a distance of 10 m. They knew that this particle was in one of the two memories, but not in which one, a common situation in the counterintuitive quantum world.

The photon would be in a state of quantum superposition in the two memories at the same time, which, surprisingly, were separated by several meters.

The team learned that entanglement had been created by detecting a photon in the telecommunications wavelength, which was also stored in quantum memories in a multiplex manner, a technique that allows several messages to be sent simultaneously over a single communication channel. These two characteristics are key to being able to scale or extend the system over large distances and, for the first time, they have been achieved together.

“Until now, other groups had already achieved several of the milestones achieved in this experiment, such as entangled quantum memories or storing photons in quantum memories with high efficiency and rate, but the uniqueness of this experiment is that our techniques have achieved it joint and efficient, and that the system can extend over great distances”, highlights Darío Lago, one of the authors of the study.

Technically, the researchers have been the first to achieve an entanglement between two solid-state quantum memories (matter-matter), with multimode properties (with different modes of propagation), remote (placed at a certain distance) and operating at the wavelength of current telecommunications.

Therefore, it is a potentially scalable technology that could be integrated into the traditional fiber optic communication network, paving the way for long-distance operations in the future quantum internet.

To carry out the experiment, the team used crystals doped with praseodymium, a chemical element from the group of so-called rare earths, as quantum memories.

Two photon pair generating sources, correlated and individual, were also used. In each pair of photons, there was one called a “messenger”, with a length within the telecommunications range of 1436 nm; and another 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).

In turn, the messenger photons were sent through an optical fiber to a device called a beam splitter, where information about their origin and path was completely erased.

Another of the authors, Samuele Grandi, comments: “We erased any kind of feature that would tell us where the messenger photons came from, because we didn’t 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 entanglement had indeed been achieved, the scientists saw a click on the monitor each time a messenger photon arrived at the detector. This entanglement was the signal photon in the superposition state between the two quantum memories, being stored as an excitation shared by tens of millions of atoms for up to 25 microseconds.

“The curious thing 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 away -Dario and Sam point out-. Although this is the main feature of our experiment, and therefore something we expected to happen, the results in the laboratory were still counterintuitive. And even more peculiar and amazing for us, we were able to control it!”

Previous studies have also experimented with entanglement and quantum memories using messenger photons to find out if entanglement between quantum memories had been successful or not.

The messenger photon acts like a carrier pigeon, and scientists can tell upon its arrival that entanglement between quantum memories has been established. When this happens, attempts to do so are stopped and the interlace is stored in memories before being analyzed.

But in this experiment a messenger photon at the telecommunications frequency is used. Therefore, the entanglement that occurs could be established with a photon compatible with existing telecommunications networks. This represents a considerable feat, as it allows long-distance entanglements to be created and these quantum technologies can be easily integrated into existing networks and classic telecommunications infrastructures.

Multiplexing in quantum repeaters
Another of the key points of the experiment has been the use of multiplexing, 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.


Dario Lago-Rivera, Samuele Grandi, Jelena V. Rakonjac, Alessandro Seri, and Hugues de Riedmatten. “Telecom-heralded entanglement between multimode solid-state quantum memories”. Nature, 2021.