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Researchers from the University of Innsbruck, funded by the US Army, have reached a record in quantum entanglement transfer between matter and light: a distance of 50 km has been traveled with fiber optic cables. The researchers said it brought the quantum internet closer. In a non-linear crystal illuminated by a powerful laser, the photon wavelength has been converted to the optimum value for long-distance displacements. Credit: IQOQI Innsbruck / Harald Ritsch
Research Triangle Park, N.C. – A search result of the US military brings the quantum internet closer. Such an Internet could offer impossible military security, detection and timing capabilities with traditional network approaches.
The distributed information center of the US Military Research Laboratory Army, funded and managed by its military research bureau, has allowed researchers from the University of the US Army to investigate the problem. Innsbruck to achieve a record in the transfer of quantum entanglement between matter and light: a distance of 50 kilometers using fiber optic cables.
The entanglement is a correlation that can be created between quantum entities such as qubits. When two qubits are entangled and one measurement is performed on one, this will affect the result of one measurement done on the other, even if that second qubit is physically far apart.
"This [50 kilometers] is two orders of magnitude farther than was possible before and is a practical distance to start building long-distance quantum networks, "said Dr. Ben Lanyon, an experimental physicist at the University of Innsbruck and principal investigator of the project, the results of which are published in the Nature journal Quantum Information (see reference below).
The inter-urban quantum networks would consist of network nodes remote from physical qubits which, despite the great physical separation, would nevertheless remain entangled. This distribution of entanglement is essential for establishing a quantum internet, researchers said.
"The demonstration is an important step towards achieving a large-scale distributed entanglement," said Dr. Sara Gamble, co-manager of the Army's Research Support Program. "The quality of the entanglement after crossing the fiber is also quite high at the other end to meet some of the toughest quantum networking application requirements."
The research team started the experiment with a atom trapped in an ion trap. With the help of laser beams, the researchers wrote a quantum state on the ion and simultaneously excited it to emit a photon in which quantum information is stored. As a result, the quantum states of the atom and the particle of light were entangled.
The challenge is to transmit the photon on fiber optic cables.
"The photon emitted by the calcium ion has a wavelength of 854 nanometers and is rapidly absorbed by the optical fiber," Lanyon said.
His team initially sent the particle of light through a nonlinear crystal illuminated by a powerful laser. The photon wavelength has been converted to the optimal value for long distance travel – the current standard telecommunication wavelength of 1,550 nanometers.
The researchers then sent this photon through a line of optical fiber 50 km long. Their measurements show that the atomic and luminous particles were still entangled even after the conversion of the wavelength and the distance traveled.
"The choice of using calcium means that these results also provide a direct path towards achieving a tangled network of atomic clocks over a great physical distance, since calcium can be co-trapped with a high-quality clock qubit. Large-scale entangled clock arrays are of great interest to the military, particularly for positioning, navigation and precision timing applications, "said Dr. Fredrik Fatemi, research scientist for the United States. Army also co-manages the program.
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The CCDC Army Research Laboratory (ARL) is an element of the US Army Combat Capabilities Command. As an Army research laboratory, ARL discovers, innovates, and makes the transition between science and technology to ensure dominant terrestrial power. Through the collaboration of basic command technical skills, the CCDC is leading the discovery, development, and provision of the technological capabilities necessary to make soldiers more effective in winning the wars of our nation and returning home safely. The CCDC is a major subordinate command of the US command of the military futures.
Reference: "Light entanglement in 50 km of optical fibers" by V. Krutyanskiy, M. Meraner, J. Schupp, V. Krcmarsky, H. Hainzer and B. P. Lanyon, August 27, 2019, npj Quantum information.
DOI: 10.1038 / s41534-019-0186-3
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