Researchers Store Entanglement across 8000 Modes in Geneva Fibre

Angelo Gelmini Rodriguez of the University of Geneva and colleagues from alto University and PSL University have stored entanglement across 16340 temporal modes, a sharp increase over previous methods. Entanglement was distributed over 5.66km of metropolitan fibre, utilising a quantum memory with a 250MHz bandwidth and storing photons for up to 63 microseconds, sharply extending the possibilities for long-distance quantum communication. The team demonstrated the storage of entanglement across 16340 temporal modes, effectively increasing the amount of information that can be transmitted by exploiting multiple spectral channels within the material.

Angelo Gelmini Rodriguez and colleagues advanced quantum communication by demonstrating the storage of entanglement across an unprecedented 16340 temporal modes, analogous to opening many more ‘lanes’ on a highway for photons to travel simultaneously. This breakthrough utilised a rare-earth-ion ensemble, a collection of atoms acting like tiny, long-lasting storage devices for quantum information, within a specially designed quantum memory.

The team successfully distributed entanglement over 5.66km of metropolitan fibre, storing photons for up to 63 microseconds and extending the reach of these delicate quantum signals, overcoming the natural degradation over distance. Detailed below is a description of the experimental setup and quantitative analysis used to achieve these results, indicating how these advancements will translate into practical, long-distance quantum networks.

Extended quantum memory entanglement facilitates high bandwidth fibre optic communication

A substantial leap beyond previous limitations of around 8000, entanglement measures now span 16340 temporal modes. This breakthrough unlocks the potential for sharply increased data transmission rates in quantum communication networks. Maintaining quantum coherence and efficiently managing the broadened bandwidth required for multiplexing previously presented challenges to storing entanglement across such many modes. Nicolas Gisin and colleagues at the University of Geneva successfully demonstrated entanglement between a telecommunication photon and a stored photon, utilising a ¹⁷¹Yb³⁺:Y₂SiO$5 crystal; its unique properties enable the simultaneous storage of multiple photonic modes.

A 25.3km fibre optic cable carried the photon, while a 979nm photon was stored within a quantum memory for 125 microseconds, successfully generating entanglement. The ¹⁷¹Yb³⁺:Y₂SiO5$ crystal exhibited a 250MHz bandwidth and a photon storage lifetime of 76.6 microseconds, parameters crucial for efficient multimode storage.

Quantifying the effective temporal mode capacity with Schmidt decomposition demonstrated storage across 16340 temporal modes, and the system was further deployed within Geneva’s fibre network, distributing entanglement over 5.66km while storing 8235 modes for 63 microseconds. Despite these advances, the demonstrated coherence lifetime of 307 microseconds remains shorter than the material’s potential 1 millisecond optical coherence time, suggesting a remaining gap before fully realised, practical long-distance quantum communication is achieved.

Metropolitan quantum networks enabled by long-lived rare-earth storage

Storing entanglement across thousands of temporal modes represents a step towards building practical quantum repeaters, devices essential for extending the range of secure quantum communication. A significant gap persists between demonstrated coherence lifetimes and the theoretical limits of the material itself, indicating that further optimisation is needed to unlock the full potential of the Yttrium orthosilicate crystal. Validating the potential of this approach for real-world networks, successful distribution of this entanglement occurred over 5.66km of metropolitan fibre, storing photons for up to 63 microseconds. Precisely controlling the absorption properties of a yttrium orthosilicate crystal doped with ytterbium ions was achieved by Nicolas Gisin and colleagues; these devices are essential for extending the distance of secure quantum communication, overcoming the inherent limitations of signal loss in fibre optic cables. Information was successfully stored within a rare-earth-ion ensemble, specifically a yttrium orthosilicate crystal, for a useful duration, though theoretical performance benchmarks remain unmet.

Storing entanglement across 16340 temporal modes demonstrates a pathway towards building quantum repeaters for long-distance communication. This achievement matters because it allows for greater data capacity and more efficient use of quantum memories, essential for overcoming signal loss in fibre optic cables.

Researchers successfully distributed entanglement over 5.66km of fibre in Geneva, storing photons for up to 63 microseconds, and they are working to extend coherence lifetimes closer to the theoretical maximum of 1 millisecond. The team quantified the effective temporal mode capacity using Schmidt decomposition to validate the performance of their ytterbium-doped yttrium orthosilicate crystal.