What is the theory?

Atomic memories are a crucial part of the quantum networking architecture because the fragile, fleeting nature of quantum information makes it extremely difficult to send logical qubits over long distances. Physical qubits cannot effectively be sent at all.

But a chain of photons – light particles – spread across a conjoined series of short network connections can transfer the logical state encoded in a single qubit. At least, it can if each pair of photons along the chain is entangled and stored in memory long enough to affect the transmission.

But entanglement is extremely sensitive to environmental noise, so photons remain paired only fleetingly and can be distributed over only short distances. This is also vital as a quantum network primes photons to transfer qubits by putting them into a state of entanglement. That creates the fundamental architecture – entangled photon pairs are distributed between the endpoints of a connection so that a qubit can be transferred between them.

Quantum networking researchers are trying to create longer connections by joining shorter hops. Quantum repeaters at adjacent junctures distribute photon pairs between them, so each pair spans a hop. The repeaters then act in unison to fuse the chain, creating a single pair of entangled photons between the endpoints of the connection, ready to transfer a qubit along it.

The process of propagating entangled pairs along every step of the connection is difficult. Each pairing is precarious, formed after countless retries, with losses inherent to the numerous procedures and components involved, so that when quantum repeaters do establish an entangled pair along a hop, they must hold it in memory until the whole chain is erected.

When the chain is then used to transmit a qubit, it is destroyed. The network is comprised of multiple chains, erected by continuous streams of photons, so that there is one ready to transfer a qubit when it is needed.

What is the reality?

So, that’s the theory behind the work done by the ICFO team: they entangled two memories and held the state for 24 microseconds – 24 millionths of a second.

“Storage time is one limitation that we are currently working on,” said Appas. “It’s not enough for a real-world implementation. In the future, we need milliseconds or even tens of milliseconds to be able to synchronise links in a multi-node network.”

Another component of the quantum repeater poses further problems: the source, which pumps out photons required to build entangled connections. ICFO made its memory from a rare earth crystal which can only store photons that have very narrow, precise bandwidth. It used a source made of optical components so large that they must sit on a desk.

Other European researchers are trying to develop sources called quantum dots, which can be integrated in chips. But their photons are 25-times too broad to be stored in the ICFO memory, said Appas. Other forms of memory crystal were “super promising”, he said, but each had its drawbacks, and the possibility of integrating them with more practical quantum dots was uncertain.

World firsts

Two other European universities have claimed “world firsts” with quantum dots, inching progress further toward a viable quantum repeater. The University of Stuttgart, along with Sapienza University of Rome, simultaneously declared that they had transmitted qubits using entangled photons emitted by two different quantum dots. Their challenge was that quantum networking processes require paired photons to be indistinguishable, while engineers have yet to fabricate dots that can emit them reliably enough for two such sources to match.

Stuttgart forced its photons to be more alike by subjecting them to a state-of-the-art frequency converter built into a 1m² rack. Having thus established a stream of photons – which is required even to attempt to create entangled pairs – it used “post-selection” to throw out those deemed unlikely to be indistinguishable.

Storage time is one limitation that we are currently working on. It’s not enough for a real-world implementation

But frequency converters are a temporary fix, said Tim Strobel, a research physicist at the Stuttgart Institute of Semiconductor Optics, while post-selection is crude and discards precious, useful photons.

“It’s just a ratio of signal to noise. Reducing the noise or increasing the signal are at the leading edge of engineering research,” said Strobel. “I’m not sure how scalable the converters are. It would be nice if we don’t need them anymore. Post-selection is a necessary tool right now but, in the future, we would want to avoid it.”

A real-world quantum repeater should be able to produce photons that are already indistinguishable, so they don’t need post-treatment, said Strobel. Much engineering must yet be done to make quantum dots capable of that. Stuttgart’s setup emitted photons that were 30% indistinguishable without post-treatment, and 75% with it.

Researchers are striving for 100% because the failures severely degrade quantum communications. Part of the solution includes a technique the Rome team used in its experiment, said Strobel.

Strain engineering

Sapienza did its experiment with a state-of-the-art quantum dot built to be physically contorted until it emits photons more reliably indistinct. Such devices are rare though and it had had only one of them, said Alessandro Laneve, research fellow at Sapienza University.

The other quantum dot in Sapienza’s experiment was an older, inferior device. Sapienza forced it to emit photons indistinguishable from the first by subjecting it to a magnetic field using equipment Laneve said would not feasibly be part of a real-world repeater. They then transmitted a qubit using photons derived from both devices. The result was “pretty good” fidelity of 82%, he said, citing a measure of how accurate the signal received was to that sent.

“It is not enough for actual quantum networking applications. We have to improve the numbers to make it useful for interfacing quantum computers or distributing quantum information reliably. We want 99.999%. But this is a first attempt, and it is already that good,” said Laneve.

Doing teleportation at all was an achievement. The next challenge was to do the much harder operation of entanglement swapping, by which a chain of entangled photons is fused to create one spanning the endpoints. For that, it was awaiting delivery of a third-generation device from Johannes Kepler University in Linz, Austria, which made the others, in collaboration with researchers in Voralberb, and in Würzburg, Germany.

The Johannes Kepler University wrote in January 2026 about how its ambition to manufacture millions of dots was tempered by the difficulty of making them emit reliably.

Laneve said: “It is a very hard challenge. But they are getting better at this, and the quality of the sources we are getting is exponentially better. There are still lots of hard technical issues, but we already know how to address some in principle. We are getting closer.”

Pushing the envelope

Sapienza got its reasonable results by doing its teleportation over a wireless link. Getting quantum dots to produce photons at a more demanding telecoms wavelength – suitable for sending over fibre-optic – is the challenge Stuttgart is trying to solve, and a second reason for its frequency conversion stop-gap.

Stuttgart and Sapienza had nevertheless pushed the envelope with these experiments. “But you have to be careful about talking about everything as a breakthrough,” said Tracy Northup, professor at the University of Innsbruck’s Institute of Experimental Physics, who is building quantum repeaters.

“There’s this march of progress. There are many groups worldwide, building on each other’s work. You see exciting results coming out every week. There are different proposals for how to build repeaters. We have we have beautiful theory papers that tell us how to build them, but experimentally they’re very challenging.

“As a community, we just don’t know what the best route will be to get to this long-term goal of a useful quantum network. So, we’re exploring different ways to get there,” she said.

The University of Innsbruck is using trapped ions (modified atoms used in some quantum computers) which can produce highly indistinguishable photons, and which it used to do entanglement swapping over a 50km link two years ago, but which has its own limitations.

Northup said the university is working with ICFO, whose memories use trapped ions, and she hopes that it might be able to incorporate the Spanish multiplexing technology in a few years. However, she added, throughput is still a big challenge for researchers.