Fig. 1.

Sketch of the experimental setup. The experiment takes place at two locations separated by 3 km. The sender is located in a radar tower of ZAMG; the receiver is the Hedy Lamarr Quantum Communication Telescope at the rooftop of our institute IQOQI. (Left) At the sender, we have a high-fidelity Sagnac-type polarization entanglement source. Whereas photon A remains in the polarization degree of freedom, photon B is transferred to OAM, using an interferometric scheme (5, 29): In it, the photon’s path is separated according to its polarization at a polarizing beam splitter (PBS) and transformed to an OAM value depending on its path using a spatial light modulator (SLM, Hamamatsu LCOS-SLM). After recombination of the paths, the transfer is completed by deleting the polarization information with a polarizer (Pol). Subsequently, the photon wave front is expanded and sent to the transceiver with a high-quality lens. Meanwhile photon A of the entangled pair is delayed in a 30-m fiber to ensure the transfer and sending of photon B before photon A is detected. After the fiber photon A is measured using a half-wave plate (λ/2) or a quarter-wave plate (λ/4)––depending on the basis in question––a PBS and two APDs. The detection times of the photons are recorded with a TTM. (Right) At the receiver, the transmitted photons are collected by a Newton-type telescope with a primary mirror of 37-cm diameter. In front of the primary mirror, opaque masks with symmetric slit patterns are used to perform mode measurements (Fig. 2). An iris (I) and a 3-nm band-pass filter (IF) were used to minimize background light. The photons are detected with an APD, and time tagged with a TTM. Coincidences are then extracted by comparing the time-tagging information from both locations.