Physicists have maintained the quantum entanglement of atoms at a record distance of 33 km

Physicists used a system of two rubidium atoms in optical traps. Quantum memory modules were

housed in two remote campus labsLudwig Maximilian University of Munich and connected by fiber optic cable. With the help of a frequency converter, scientists managed to maintain quantum entanglement at a record distance.

The laser pulse excites rubidium atoms, afterwhich they spontaneously return to their ground state, each emitting a photon. Due to the conservation of angular momentum, the spin of an atom is entangled with the polarization of the photon it emits. Scientists have used these particles to quantum-mechanically connect two atoms. They transmitted them over a fiber optic cable to a receiving station, where a joint measurement of the photons reveals quantum entanglement.

The difficulty of transmitting such a signal to a largethe distance is related to the wavelength of the emitted radiation. Most of the elements used as quantum memory emit light in the visible or near infrared range. In glass fibers, such photons can travel about a kilometer and then get lost, the scientists explain.

Scheme of the experimental setup. The frequency-converted photons are transmitted to a central station, where the Bell state meter changes entanglement. Source: Tim van Leent et al, Nature

To overcome this limitation, physicistsoptimized the wavelength of photons transmitted over the network. Using two quantum frequency converters, they increased the original wavelength from 780 nm to 1517 nm. At the same time, the researchers managed to achieve an unprecedented conversion efficiency: 57%.

Our experiment is special in that wewe really entangle two stationary particles, that is, atoms that act as quantum memory. This is much more difficult than photon entanglement, but opens up many other possible applications.

Tim van Lint, physicist at the Ludwig Maximilian University of Munich, co-author of the publication

The researchers believe that the frequency conversion system will help in building large-scale quantum networks and creating secure quantum communication protocols.

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