Entanglement Over 50 km of Optical Fiber

Illustration of a light particle entangled with matter over 50 km of optical fiber. (Source: H. Ritsch, IQOQI)

For the first time, a team led by Innsbruck physicist Ben Lanyon has sent a light particle entangled with matter over 50 km of optical fiber. This paves the way for the practical use of quantum networks and sets a milestone for a future quantum internet. The quantum internet promises abso­lutely tap-proof communication and powerful dis­tributed sensor networks for new science and tech­nology. However, because quantum infor­mation cannot be copied, it is not possible to send this information over a classical network.

Quantum information must be transmitted by quantum particles, and special interfaces are required for this. Ben Lanyon is researching these important inter­sections of a future quantum Internet. Now his team at the Department of Experi­mental Physics at the University of Innsbruck has achieved a record for the transfer of quantum ent­anglement between matter and light. For the first time, a distance of fifty 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.

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.

As a next step, Lanyon and his team show that their methods would enable entanglement to be generated between ions 100 kilo­meters apart and more. Two nodes send each an entangled photon over a distance of 50 kilometers to an inter­section where the light particles are measured in such a way that they lose their entangle­ment with the ions, which in turn would entangle them. With 100-kilometer node spacing now a possi­bility, one could therefore envisage building the world’s first intercity light-matter quantum network in the coming years: only a handful of trapped ion-systems would be required on the way to establish a quantum internet between Innsbruck and Vienna, for example. Lanyon’s team is part of the Quantum Internet Alliance, an inter­national project within the Quantum Flagship framework of the European Union. (Source: IQOQI)

Reference: V. Krutyanskiy et al.: Light-matter entanglement over 50 km of optical fibre, npj Quantum Inf. 5, 72 (2019); DOI: 10.1038/s41534-019-0186-3

Link: Quantum Optics and Spectroscopy (R. Blatt), University Innsbruck, Austria

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