A network in which data trans­mission is perfectly secure against hacking? If physicists have their way, this will become reality one day with the help of the quantum ent­anglement. A team led by Harald Weinfurter from University of Munich (LMU) and Christoph Becher from Saarland University have now coupled two atomic quantum memories over a 33-kilo­meter long fiber optic connection. This is the longest distance so far that anyone has ever managed entangle­ment via a telecom fiber. The quantum mechanical entangle­ment is mediated via photons emitted by the two quantum memories. A decisive step was the researchers’ shifting of the wavelength of the emitted light particles to a value that is used for conven­tional telecommuni­cations. “By doing this, we were able to signi­ficantly reduce the loss of photons and create entangled quantum memories even over long distances of fiber optic cable,” says Weinfurter.

Generally speaking, quantum networks consist of nodes of individual quantum memories – such as atoms, ions, or defects in crystal lattices. These nodes are able to receive, store, and transmit quantum states. Mediation between the nodes can be ac­complished using light particles that are exchanged either over the air or in a targeted manner via fiber optic connec­tion. For their experiment, the researchers use a system comprised of two optically trapped rubidium atoms in two labora­tories on the LMU campus. The two locations are connected via a 700-meter-long fiber optic cable, which runs under­neath Geschwister Scholl Square in front of the main building of the university. By adding extra fibers on coils, connections of up to 33 kilometers in length can be achieved.

A laser pulse excites the atoms, after which they spontaneously fall back into 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 its 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 through the fiber optic cable to a receiver 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 make it just a few kilometers before they are lost,” explains Christoph Becher. For this reason, 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 around 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 ac­complished the conversion with an unpre­cedented effi­ciency of 57 percent. At the same time, they managed to preserve the quality of the information stored in the photons to a high degree, which is a condition of quantum coupling.

“The significance of our experiment is that we actually entangle two stationary particles – that is to say, atoms that function as quantum memories,” says Tim van Leent. “This is much more difficult than ent­angling photons, but it opens up many more application possi­bilities.” The researchers think that the system they developed could be used to construct large-scale quantum networks and for the implementation of secure quantum communi­cation protocols. “The experiment is an important step on the path to the quantum internet based on existing fiber optic infra­structure,” says Harald Weinfurter. (Source: LMU)

Reference: T. van Leent et al.: Entangling single atoms over 33 km telecom fibre, Nature 607, 69 (2022); DOI: 10.1038/s41586-022-04764-4

Link: Quantum Communication, Munich Center for Quantum Science and Technology, Munich, Germany