Researchers from the University of Innsbruck and at the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences have achieved a record for the transfer of quantum entanglement over a distance of 50 kilometers using fiber optic cables.
The photon emitted by a calcium ion has a wavelength of 854 nm and would be quickly absorbed by the optical fiber. So the idea was to send the photon through a nonlinear crystal illuminated by a strong laser. Thereby the photon wavelength is converted to the current telecommunications standard wavelength of 1550 nm. (Phys.org)
