Record-breaking entanglement project brings us closer to a quantum internet

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Close- up image of a rare-earth doped crystal used as a quantum memory. (©ICFO)

The coming world of quantum technology will require an internet to transfer data within the quantum domain, and society is well on its way to achieving such a network because researchers have successfully entangled quantum memories at a record distance and for the longest duration to date.

"It's 40 times higher than the record for long-lived entanglement," study author Samuele Grandi told The Academic Times. "They could keep the entanglement for only some nanoseconds — we are in the microsecond regime."

Grandi and his team entangled the nodes in separate labs 10 meters apart and upheld the state for a maximum of 25 microseconds. Their method incorporated telecommunication channels that already exist, ensuring that the system can one day be implemented easily and inexpensively. 

Their method, described in a paper published June 2 in Nature, is based on a concept called a quantum repeater, which serves as a stepping stone during a long-distance entanglement process that protects the signal during travel.

"Connecting quantum computers is the reason behind the quantum internet — and quantum repeaters [are] at the base," Grandi said.

Quantum computers are expected to expedite drug development by several years, serve ultrasecure cryptographic purposes and perform other feats that are unrealistic with current technology. But these devices will require a special network to share information because they yield data stored in quantum bits, or qubits. Qubits follow quantum laws, in that they exist in superposition, which means they exist in a state of being and not being at the same time.

The concept can be imagined by thinking of a standard six-sided die. While someone rolls it, the die represents six different outcomes — or states — simultaneously. After it's thrown on a table, however, the number on top becomes its new nature and has only one state, so its "superposition" has been interrupted.

The picture is counterintuitive, but that's because a die isn't a quantum particle. This is what actually happens in the quantum domain, and qubits are no exception. The difference is that qubits exist in a state of 0 and 1; a string of qubits is like a quantum computer sentence.

For the experiment, Grandi, a postdoctoral researcher at the Institute of Photonic Sciences at the Barcelona Institute of Science and Technology, and his team used photons — which are also quantum particles — in the place of qubits. They generated them with a standard technique involving two crystals in different laboratories separated by 10 meters. 

"Being in the lab, and being able to see [that] this is actually how nature works, and [to] be able to control it and to adjust the parameters — that was something I've enjoyed particularly," Grandi said.

Strikingly, the team's procured particle pairs were also entangled. Returning again to the six-sided die example, imagine that the situation has now been modified to include two dice. If it were the case that the first die's rolling of a four meant that the second die would also roll a four, the outcomes of the two dice would be entangled. 

Even if one could see the outcome of only the first die, some information would be learned about the second die's state. This is also true for quantum particles, even if they are on opposite ends of the universe.

Entanglement is the at crux of the quantum internet, so in order to create a foundation that can share quantum data without interrupting a particle's superposition, the researchers played around with entangled particles. While it is relatively straightforward to isolate a quantum particle and send it a short distance, and while many researchers have done so in the race toward building quantum computers, longer distances pose a more complex problem. 

"[A classical signal] gets lower and lower in intensity as it goes through fibers or cables," Grandi said, but "we can just amplify them again, and we just repeat it — but you cannot do that with quantum signals."

Any type of signal, in superposition or not, starts to get lost while traveling; that's why cell-phone reception tends to be poor in remote areas. Quantum signals cannot be amplified, and attempting to do so would ruin the message, because any type of noise interrupts the system. 

If one tries to send a single photon beyond about 100 kilometers, Grandi said, "you have a 1% chance of having it on the other side."

A quantum repeater is the workaround, acting as a relay station for quantum particles.

Rather than try a solution destined to fail, Grandi worked with the quantum repeater concept. Instead of sending one photon, holding quantum information, along the entire distance until it reaches another to entangle with, the team took advantage of their ability to make pre-entangled particle pairs.

Within the team's entangled photon pairs, each generated by the spaced-apart crystals in one of two labs, one particle was compatible with the node itself, or quantum memory, and the other was in the telecommunication band, meaning it was mobile.

The particle from each node that was associated with the telecom band traveled to a junction midway at a mirror with a 50% probability of reflection, which is called a beamsplitter. Upon interacting with each other, the two departed particles became entangled without any interruption of superposition.

"If, after this beamsplitter, we see one photon, we do not know where it came from," Grandi explained, "because it could have equally come from node A or node B."

Another recent publication from Chinese researchers, released in the same journal issue as Grandi's, used a similar mechanism, except the team observed two photons at the center point rather than one.

In Grandi's experiment, however, because each photon was already entangled with its initial pair, those particles were forced to adjust accordingly as well — the long-distance nodes, or quantum memories, became fully entangled and ready for more information, which would be given rather instantaneously because the associated particles remained dependent on each other.

Ultimately, quantum repeaters act like station checkpoints for a messenger dove particle. So if a chain of these repeaters could be created, we would have a functional quantum internet that could be spread across the globe.

The paper, "Telecom-heralded entanglement between remote multimode solid-state quantum memories," was published June 2 in Nature. It was authored by Dario Lago-Rivera, Samuele Grandi, Jelena V. Rakonjac and Alessandro Seri, the Barcelona Institute of Science and Technology; and Hugues de Riedmatten, The Barcelona Institute of Science and Technology and Institució Catalana de Recerca i Estudis Avançats.

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