Record the entanglement of quantum memories

Researchers from LMU and Saarland University have entangled two quantum memories via a 33-kilometer-long fiber-optic connection — a record and an important step towards the quantum internet.

A network where data transmission is highly secure against hacking? If physicists succeed, this will one day become a reality with the help of the quantum mechanical phenomenon known as entanglement. For entangled particles, the rule is: If you measure the state of one particle, you automatically know the state of the other. It makes no difference how far the particles are entangled with each other. These are ideal circumstances for transmitting information over long distances in a way that makes eavesdropping impossible.

A team led by physicist Prof. Harald Weinfurter from LMU and Prof. Christoph Becher of the University of Saarland has now combined two atomic quantum memories via a 33-kilometer-long fiber-optic link. This is the furthest distance anyone has ever managed via telecommunications fiber. Quantum mechanical entanglement is mediated through photons emitted by two quantum memories. The decisive step was for researchers to shift the wavelength of the emitted light particles to a value used for conventional telecommunications. “By doing this, we were able to significantly reduce photon loss and create entangled quantum memories even over long-distance fiber optic cables,” Weinfurter said.

In general, a quantum network is made up of individual quantum memory nodes – such as atoms, ions, or defects in a crystal lattice. These nodes can receive, store, and transmit quantum states. Mediation between nodes can be achieved using light particles exchanged either through the air or in a targeted manner via fiber optic connections. For their experiment, the researchers used a system consisting of two optically trapped rubidium atoms in two laboratories on the LMU campus. The two sites are connected via a 700-meter-long fiber optic cable, which runs under Geschwister Scholl Square in front of the university’s main building. By adding extra fiber to the coils, splices of up to 33 kilometers can be achieved.

A laser pulse excites the atoms, after which they spontaneously fall back to their ground state, each thereby emitting a photon. Due to the conservation of angular momentum, the spin of the atom is entangled with the polarization of the photon it emits. 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 fiber-optic cable to a receiving station, where the co-measurements of the photons demonstrated quantum memory entanglement.

However, most quantum memories emit light with wavelengths in the visible or near infrared range. “In optical fiber, these photons make it only a few kilometers before they disappear,” explains Christoph Becher. For this reason, physicists from Saarbrücken and his team optimized the wavelengths of photons for their travels down 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 telecommunications wavelength of about 1,550 nanometers,” Becher said. The telecommunications band is the frequency range over which light transmission in optical fiber has the lowest loss. Becher’s team completed the conversion with an unprecedented 57 percent efficiency. At the same time, they managed to maintain the quality of the information stored in the photons to a high level, which is a state of quantum coupling.

“The significance of our experiment is that we actually entangle two stationary particles — that is, atoms that serve as quantum memories,” said Tim van Leent, lead author of the paper. “It’s much more difficult than entangling photons, but it opens up more application possibilities.” The researchers think that the system they developed could be used to build large-scale quantum networks and for the implementation of secure quantum communication protocols. “This experiment is an important step towards a quantum internet based on the existing fiber optic infrastructure,” said Harald Weinfurter.

Reference:

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. Involving a single atom over 33 km of telecommunications fiber. Nature, 2022; 607 (7917): 69 DOI: 10.1038/s41586-022-04764-4

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