Record entanglement of quantum memories

Researchers from LMU and Saarland University have entwined two quantum memories over a 33-kilometer fiber connection – a record and an important step towards the quantum internet.

A network in which data transmission is perfectly protected against hacking? If physicists have their way, this will one day become a reality using the quantum mechanical phenomenon known as entanglement. For entangled particles, the rule is: if you measure the state of one of the particles, you automatically know the state of the other. It doesn’t matter how far apart the entangled particles are. This is an ideal state of affairs for transmitting information over long distances in a way that makes eavesdropping impossible.

A team led by physicists Prof. Harald Weinfurter from LMU and Prof. Christoph Becher from the University of Saarland have now linked two atomic quantum memories over a 33-kilometer fiber link. This is the longest distance to date anyone has ever managed to entangle over a telecom fiber. The quantum mechanical entanglement is mediated through photons emitted from the two quantum memories. A decisive step was the shifting of the wavelength of the emitted light particles by the researchers to a value that is used for conventional telecommunications. “By doing this, we were able to significantly reduce photon loss and create entangled quantum memories even over long distances of fiber optic cable,” Weinfurter says.

In general, quantum networks consist of nodes of individual quantum memories – such as atoms, ions or defects in crystal lattices. These nodes can receive, store and transmit quantum states. Mediation between the nodes can be done with light particles that are exchanged either via the air or in a targeted manner via a fiber optic connection. For their experiment, the researchers use a system consisting of two optically confined rubidium atoms in two labs on the LMU campus. The two locations are connected via a 700-metre long fiber optic cable, which runs under Geschwister Schollplein in front of the main university building. By adding extra fibers on spools, connections up to 33 kilometers in length can be realized.

A laser pulse excites the atoms, after which they spontaneously fall back to their ground state, each emitting a photon. Due to the conservation of angular momentum, the spin of the atom is entangled with the polarization of the emitted photon. These light particles can then be used to create a quantum mechanical coupling of the two atoms. To do this, the scientists sent them via the fiber optic cable to a receiving station, where a joint measurement of the photons indicates an entanglement of the quantum memories.

However, most quantum memories emit light with wavelengths in the visible or near infrared range. “In fiber optics, these photons only make it a few kilometers before they are lost,” explains Christoph Becher. Therefore, the physicist from Saarbrücken and his team optimized the wavelength of the photons for their journey in the cable. Using two quantum frequency converters, they increased the original wavelength from 780 nanometers to a wavelength of 1,517 nanometers. “This is close to the so-called telecom wavelength of about 1,550 nanometers,” says Becher. The telecom band is the frequency range in which the transmission of light in fiber optics has the lowest losses. Becher’s team achieved the conversion with an unprecedented efficiency of 57 percent. At the same time, they managed to maintain a high degree of quality of the information stored in the photons, which is a prerequisite for quantum coupling.

“The significance of our experiment is that we are actually entangling two stationary particles — that is, atoms that act as quantum memories,” said Tim van Leent, lead author of the paper. “This is much more difficult than photon entanglement, but it offers much more application possibilities.” The researchers believe that the system they have developed can be used to build large-scale quantum networks and to implement secure quantum communication protocols. “The experiment is an important step towards the quantum internet based on existing fiber optic infrastructure,” says Harald Weinfurter.


  1. Tim van Leent, Matthias Bock, Florian Fertig, Robert Garthoff, Sebastian Eppelt, Yiru Zhou, Pooja Malik, Matthias Seubert, Tobias Bauer, Wenjamin Rosenfeld, Wei Zhang, Christoph Becher, Harald Weinfurter. Entanglement of single atoms over 33 km of telecom fiber. Nature, 2022; 607 (7917): 69 DOI: 10.1038/s41586-022-04764-4
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