Quantum Entanglement Sent Over 50 Km of Optical Fiber

A team from the University of Innsbruck and the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences has achieved what could be a record for the transfer of quantum entanglement between matter (a trapped ion) and light (a photon). The team sent quantum information over a distance of 50 km using fiber optic cables. “This is two orders of magnitude further than was previously possible and is a practical distance to start building intercity quantum networks,” lead researcher Ben Lanyon said.

The researchers began by trapping a calcium atom in an ion trap. Using lasers, they wrote a quantum state onto the ion while exciting it to emit a photon, in which quantum information was stored. The quantum states of the atom and the photon became entangled.

The photon that was emitted by the calcium ion had a wavelength of 854 nm. To prevent the photon from being absorbed by the optical fiber as it was being transmitted over a fiber optic cable, the researchers first sent the photon through a nonlinear crystal illuminated by a strong laser. This step caused the photon wavelength to be converted to 1550 nm, the current telecommunications standard wavelength and a suitable wavelength for long-distance travel.

In a nonlinear crystal illuminated by a strong laser, the photon wavelength is converted to the optimal value for long-distance travel. Courtesy of IQOQI Innsbruck/Harald Ritsch.

The researchers then sent the photon through a 50-km-long optical fiber line. Their measurements showed that the atom and photon were still entangled, even after the wavelength conversion and the 50-km journey.

Next, the researchers plan to show that their method could be used to enable entanglement between ions 100 km apart and more. With the possibility of 100-km node spacing, building the world’s first intercity light-matter quantum network could be possible in the coming years. For example, just a small number of trapped ion-systems would be required to establish a quantum internet between Innsbruck and Vienna, the team said.

The research was published in npj Quantum Information (https://doi.org/10.1038/s41534-019-0186-3).