New record for entangled state transport

Physicists at Ludwig-Maximilians-Universitaet (LMU) in Munich, together with colleagues at Saarland University, have successfully demonstrated the transport of an entangled state between an atom and a photon via an optic fiber over a distance of up to 20 km — thus setting a new record.

The team has shown that the entangled state of an atom and a photon can be transmitted via an optic fiber (like those used in telecommunications networks) over a distance of up to 20 km. The previous record was 700 meters.

In their experiment, the researchers entangled a rubidium atom with a photon, and were able to detect the entangled state after its passage through a 20-km coil of optic fiber. The rubidium atom emits photons with a wavelength of 780 nanometers, a wavelength is rapidly absorbed in an optic fiber made of glass. Conventional telecommunications networks therefore make use of wavelengths around 1550 nanometers, which reduces losses in transit.

So the team built what is called a quantum frequency converter that was specifically designed to increase the wavelength of the emitted photons from 780 to 1520 nanometers. That task is really challenging because the system must ensure that conversion from only a single photon to only one other photon happens and that none of the other properties of the entangled state, especially the polarization of the photon, has been altered during the conversion process.

This work constitutes a crucial step toward the future realization of quantum networks. (LMU)

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