⚛️ quantum physics

Entanglement distribution and quantum storage of more than 8000 modes over a metropolitan network

This paper demonstrates a quantum repeater node utilizing a multimode rare-earth-ion memory to successfully distribute entanglement across a metropolitan fiber network while storing over 8,000 temporal modes, marking a significant step toward practical, high-capacity quantum communication.

Original authors: Angelo Gelmini Rodriguez, Louis Nicolas, Théo Sanchez Mejia, Pavel Sekatski, Nicolas Brunner, Towsif Taher, Rob Thew, Philippe Goldner, Mikael Afzelius

Published 2026-08-14

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CC BY 4.0

Original authors: Angelo Gelmini Rodriguez, Louis Nicolas, Théo Sanchez Mejia, Pavel Sekatski, Nicolas Brunner, Towsif Taher, Rob Thew, Philippe Goldner, Mikael Afzelius

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the internet as a giant, bustling highway where information zips around at the speed of light. Now, imagine trying to build a super-highway for a completely new kind of traffic: "quantum" information. This isn't just about sending emails faster; it's about creating a network that is fundamentally unhackable and capable of solving problems that would take today's supercomputers millions of years. To make this happen, scientists are trying to build "quantum repeaters." Think of these like rest stops on a long road trip. Because quantum signals are incredibly fragile, they can't travel very far without fading away or getting lost. A repeater catches the signal, stores it safely, and then sends it on its way to the next stop. But here's the tricky part: the signal travels so fast that by the time the repeater figures out how to store it, the signal has already zoomed past. To fix this, the repeater needs to be able to catch many signals at once, like a net that can scoop up a whole school of fish instead of just one. This is called "multiplexing," and it's the secret sauce needed to make a real, working quantum internet.

In this exciting new study, a team of researchers from Switzerland and Finland has built a prototype of one of these high-tech rest stops and tested it in the real world. They created a special "quantum memory" made from a crystal doped with rare-earth ions (specifically Ytterbium-171). You can think of this crystal as a magical sponge that can soak up light particles (photons) and hold them for a tiny fraction of a second before spitting them back out. The team didn't just store one photon; they managed to store thousands of them at the same time, effectively turning a single-lane road into a massive multi-lane superhighway.

The researchers set up their experiment in two ways. First, in their lab, they sent a photon through a long coil of fiber optic cable (25.3 kilometers) and stored its partner photon in their crystal memory. They proved that the two photons remained "entangled"—a spooky quantum connection where they share a fate regardless of distance—even after the storage. But the real magic happened when they took this setup out of the lab and into the city of Geneva. They connected their quantum memory to a detector in a different university building using a 5.66-kilometer stretch of the actual city's fiber optic network.

The results were impressive. In the lab, they successfully stored and retrieved entanglement across 16,340 different "time slots" (or modes) at once. When they moved to the city network, they still managed to store a staggering 8,235 modes for 63 microseconds. To put that in perspective, previous experiments had managed to store only a few dozen or maybe a few hundred modes. By using a mathematical tool called "Schmidt decomposition," the team showed that their memory is incredibly efficient at handling this massive amount of data simultaneously. They also confirmed that the entanglement survived the journey through the city cables, proving that this technology can work outside a controlled lab environment.

While this isn't a fully functional quantum internet yet, it's a massive leap forward. The paper suggests that by using this specific type of crystal and memory technique, we can overcome the biggest bottleneck in quantum networking: the speed limit caused by the time it takes for light to travel. The researchers note that while their current system is a huge improvement, there is still room to grow. They point out that the memory's "holding time" could potentially be extended from microseconds to milliseconds, and the efficiency could be boosted further. For now, though, they have demonstrated that it is possible to build a quantum repeater node that can catch, hold, and release thousands of quantum signals at once, paving the way for a future where quantum communication is not just a dream, but a reality running right under our feet in the city's fiber cables.

Technical Summary: Entanglement Distribution and Quantum Storage of More Than 8000 Modes over a Metropolitan Network

Problem Statement
The realization of practical fiber-based quantum repeaters is hindered by rate limitations imposed by photon propagation time in optical fibers (approximately 5 µs/km). When a quantum memory can store only a single excitation (mode), the entanglement generation rate is severely constrained. To overcome this, multiplexing is required to allow many entanglement-generation attempts within a single round-trip time. While ensemble-based quantum memories offer the potential for multimode storage across temporal, frequency, or spatial domains, demonstrating high-capacity storage of quantum entanglement in a metropolitan environment remains a significant challenge.

Methodology
The authors present a quantum repeater node integrating a broadband multimode quantum memory with an entangled photon-pair source.

Quantum Memory: The core component is a $^{171}\text{Yb}^{3+}:\text{Y}_2\text{SiO}_5$ crystal doped with 2 ppm of ions, cooled to ~3 K. The memory utilizes the Atomic Frequency Comb (AFC) technique, creating a comb-like modulation of the absorption profile with a periodicity $\Delta$ . This allows for the storage of a photon for a duration $1/\Delta$ . The system exploits the hybridized electron-nuclear hyperfine states of $^{171}\text{Yb}^{3+}$ , which provide long optical coherence times and large hyperfine splittings, enabling a bandwidth of 250 MHz.
Photon Source: An entangled photon-pair source based on Spontaneous Parametric Downconversion (SPDC) in a periodically poled lithium niobate (PPLN) waveguide. A 600 nm pump generates photon pairs at 979 nm (signal) and 1553 nm (idler, telecom C-band).
Bandwidth Matching: The 1553 nm idler is filtered by a 90 MHz Fabry-Pérot cavity and a 75 GHz DWDM filter. The 979 nm signal is filtered by a 20 GHz Volume Bragg Grating (VBG) and the 250 MHz AFC memory itself.
Experimental Sequence: The protocol involves a preparation phase (spin polarization and AFC burning) followed by a measurement phase. The SPDC pump duration is matched to the storage time ( $T_{pump} = 1/\Delta$ ) to maximize temporal modes. The telecom photon propagates through fiber spools or a metropolitan network, while the 979 nm photon is stored and subsequently retrieved via the AFC echo.
Multimode Analysis: The effective temporal mode capacity is quantified using a Schmidt decomposition of the Joint Temporal Amplitude (JTA) of the two-photon state. The authors utilize the measured cross-correlation function $g^{(2)}_{si}(\tau_{si})$ to calculate a lower bound for the Schmidt mode number $N$ .
Entanglement Verification: Energy-time entanglement is verified using a Franson-type interferometer setup. The telecom photon passes through a phase-stabilized fiber Michelson interferometer, while the 979 nm photon is analyzed using a double-AFC scheme. A Clauser-Horne-Shimony-Holt (CHSH) Bell test is performed to witness entanglement.

Key Contributions

High-Capacity Storage: The demonstration of quantum storage across a record number of temporal modes. The system achieved a storage of 16,340 temporal modes for a storage time of 125 µs in a laboratory setting.
Quantitative Metric: The introduction and validation of a quantitative measure for effective temporal mode capacity using Schmidt decomposition, specifically calculating a lower bound ( $N$ ) based on experimental cross-correlation data.
Field Deployment: The successful distribution of entanglement over a 5.66 km standard single-mode fiber link within the Geneva metropolitan network. In this field test, the system stored 8,235 modes for 63 µs.
Performance Characterization: Detailed characterization of the memory efficiency, which decays exponentially with a characteristic memory time of $76.6 \pm 11.0$ µs, corresponding to an effective AFC coherence lifetime of $307 \pm 44$ µs.

Results

Laboratory Fiber Spools: Entanglement was successfully distributed and stored across fiber lengths ranging from 0.5 km to 25.3 km. The CHSH parameter $S$ violated the classical inequality ( $S \le 2$ ) for all distances, with values ranging from $2.65$ to $2.73$. The number of stored modes scaled with storage time, reaching 16,340 modes at 125 µs.
Metropolitan Network: In the field test connecting two university buildings (2.15 km straight-line distance, 5.66 km fiber), the system achieved $S = 2.65 \pm 0.04$ for 32 µs storage (4,183 modes) and $S = 2.67 \pm 0.04$ for 63 µs storage (8,235 modes).
Efficiency and Rates: The memory efficiency at short storage times was measured at $(19 \pm 1)\%$ . While the coincidence detection rate decreased with longer storage times due to exponential decay, the system maintained high cross-correlation values ( $\bar{g}^{(2)}_{si}$ ) and visibility, indicating preserved entanglement quality.

Significance and Claims
The paper claims that these experiments constitute "half of an elementary quantum repeater link," demonstrating the critical capability of storing and retrieving entanglement in a multimode regime over realistic distances. The authors emphasize that the high multimode capacity (up to 16,340 modes) represents an improvement of one to two orders of magnitude over previous works, which is essential for achieving practical entanglement generation rates in quantum repeaters.

The authors modestly note that while the Schmidt mode number provides a theoretical speed-up factor for a simplified model, the actual speed-up in a full repeater scheme depends on additional parameters such as pump power and detection windows. They identify future work as necessary to extend storage durations to the millisecond regime (via spin-wave AFC) and to improve efficiency, which would be required for a fully scalable quantum repeater based on this platform. The work highlights the unique advantages of $^{171}\text{Yb}^{3+}:\text{Y}_2\text{SiO}_5$ in combining large bandwidth with long optical coherence times.

Technical Summary: Entanglement Distribution and Quantum Storage of More Than 8000 Modes over a Metropolitan Network

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