A quantum repeater node built with a ytterbium-doped crystal has stored entanglement across more than 8,000 temporal modes while distributing photons through 5.66 kilometers of standard metropolitan fiber in Geneva, Switzerland.
The experiment, described in a preprint posted on arXiv on August 13 by researchers led by a team at the University of Geneva, paired a 171Yb3+:Y2SiO5 multimode quantum memory with a bandwidth-matched entangled photon-pair source operating at telecom wavelengths. The memory offered a 250 MHz bandwidth and a 76.6 microsecond storage lifetime. In a controlled lab setting with a 25.3 kilometer fiber spool, the team demonstrated entanglement between a telecom photon and a 979 nm photon stored for 125 microseconds across 16,340 temporal modes. They then took the setup outside, threading signal photons through the Geneva metropolitan fiber network while storing 8,235 modes for 63 microseconds. The team introduced a quantitative measure of effective temporal mode capacity using a Schmidt decomposition, giving the field a standardized way to compare multimode quantum memories.
Practical quantum networks need high communication rates, and multiplexing across many modes is how you get there without laying dedicated fiber. Recent tests sending entangled photons through sixty-plus kilometers of outdoor fiber have already shown that quantum links can survive real-world conditions. The gap those experiments left was storage: keeping entanglement alive long enough to synchronize signals across a network. The Geneva deployment starts to close that gap. The result also adds context to the race to build quantum hardware that outperforms classical supercomputers by showing that the infrastructure side of quantum communication is advancing just as fast as the processor side.
Data on the entanglement fidelity achieved in the metropolitan deployment was not specified in the abstract.