Entanglement sent over 50 km of optical fiber
For the first time, a distance of 50 kilometers was covered using fiber optic cables. "This is two orders of magnitude further than was previously possible and is a practical distance to start building inter-city quantum networks," says Ben Lanyon.
Converted photon for transmission
Lanyon's team started the experiment with a calcium atom trapped in an ion trap. Using laser beams, the researchers write a quantum state onto the ion and simultaneously excite it to emit a photon in which quantum information is stored. As a result, the quantum states of the atom and the light particle are entangled. But the challenge is to transmit the photon over fiber optic cables. "The photon emitted by the calcium ion has a wavelength of 854 nanometers and is quickly absorbed by the optical fiber," says Ben Lanyon. His team therefore initially sends the light particle through a nonlinear crystal illuminated by a strong laser. Thereby the photon wavelength is converted to the optimal value for long-distance travel: the current telecommunications standard wavelength of 1550 nanometers. The researchers from Innsbruck then send this photon through a 50-kilometer-long optical fiber line. Their measurements show that atom and light particle are still entangled even after the wavelength conversion and this long journey.
See the full story here: https://phys.org/news/2019-08-entanglement-km-optical-fiber.html