Quantum Entanglement Sent Over 50 Km of Optical Fiber

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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.

Quantum entanglement between light and matter sent over 50 km of optical fiber, University of Innsbruck.
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 ( 

Published: August 2019
quantum optics
The area of optics in which quantum theory is used to describe light in discrete units or "quanta" of energy known as photons. First observed by Albert Einstein's photoelectric effect, this particle description of light is the foundation for describing the transfer of energy (i.e. absorption and emission) in light matter interaction.
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.
Research & TechnologyeducationEuropeUniversity of Innsbruckfiber opticsLasersLight SourcesOpticsnonlinear opticsquantum opticssingle photons and quantum effectsphoton entanglementCommunicationsquantum communicationsquantum InternetBen Lanyonoptical fiberTech Pulse

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