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Quantum Age -Entanglement Sent Over 50 Km Of Optical Fiber

The quantum internet shows the possibility of absolutely tap-proof communication and powerful distributed sensor networks for innovative science and technology. As quantum information cannot be copied, it cannot to transmitted over a classical network. Such information must be transmitted by quantum particles via special interfaces. The experimental physicist Ben Lanyon who is based inĀ  Innsbruck, was awarded the Austrian START Prize in 2015 for his research, is currently investigating these imperative intersections of a future quantum Internet.

His efforts have borne fruit as his team at the Department of Experimental Physics at the University of Innsbruck and at the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences have achieved a record transfer of quantum entanglement between matter and light. For the first time ever, a distance of 50 kilometers was covered with fiber optic cables and the current results were published in the Nature journal Quantum Information.

Lanyon’s team is part of the Quantum Internet Alliance, an international project within the Quantum Flagship framework of the European Union.

“This is two orders of magnitude further than was previously possible and is a practical distance to start building inter-city quantum networks,” shares Ben Lanyon.

Converted Photon For Transmission

Lanyon’s team initiated their experiment with a calcium atom enclosed in an ion trap. The researchers used laser beams to write a quantum state onto the ion and at the same time excite it to emit a photon which carried the quantum information. Consequently, the quantum states of the atom and the light particle got entangled. But the main challenge was to transmit the photon over fiber optic cables.

Ben Lanyon explained, “The photon emitted by the calcium ion has a wavelength of 854 nanometers and is quickly absorbed by the optical fiber”.

Therefore, his team first sent the light particle through a nonlinear crystal illuminated by a strong laser. By this means the photon wavelength achieved the optimal value for long-distance travel: the present telecommunications standard wavelength of 1550 nanometers. The Innsbruck researchers then send this photon through a 50-kilometer-long optical fiber line. Their measurements confirmed that atom and light particle were still entangled even after the wavelength conversion and the 50km long journey.

Even Greater Distances In Sight

In the next step, Lanyon and his team showed that their methods could enable the generation of entanglement between ions 100 kilometers apart and more. Each of the two nodes sends an entangled photon over a distance of 50 kilometers to an intersection where the light particles are measured in a particular way that they lose their entanglement with the ions, which in line would entangle them. With 100-kilometer node spacing now a prospect, one could thus envisage building the world’s first intercity light-matter quantum network in the coming future. For example, only a handful of trapped ion-systems would be required en-route to establish a quantum internet between Innsbruck and Vienna.

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