With the advance of optical high-speed internet connections, a dense network of glass fibers has been established throughout Europe. Although they were not planned to be used for quantum communication, it is straightforward to send quantum states along such fibers. IQOQI is therefore preparing to run the first border-crossing quantum experiments on telecom fibers, which will be a huge step towards the goal of a "quantum internet". Central and Eastern Europe are the ideal testbed for experiments like this due to the relatively short distances between metropolitan areas and important economical and political hubs such as Munich, Bratislava, Budapest, Prague and of course Vienna.
The telecommunication fibers are predominantly designed for light of 1550 nm wavelength where absorption losses are lowest. Recent experiments have shown that such fiber links are also usable for transmission of entangled photons over long distances without having to rely on so-called "trusted nodes". This is possible by developing an ultra-bright waveguide-based source of polarization-entangled photons which will generate up to a billion photon pairs per second. These pairs are spatially separated and coupled into telecom fibers, which will e.g. connect Vienna, Bratislava and Budapest in a first step. There, the photons will be read out and have their polarization analyzed by a detector scheme including highly efficient (>80%) superconducting nanowire single-photon detectors (SNSPDs).
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