title: Twisted photon entanglement through turbulent air across Vienna.
abstract: Photons with a twisted phase front can carry a discrete, in principle, unbounded amount of orbital angular momentum (OAM). The large state space allows for complex types of entanglement, interesting both for quantum communication and for fundamental tests of quantum theory. However, the distribution of such entangled states over large distances was thought to be infeasible due to influence of atmospheric turbulence, indicating a serious limitation on their usefulness. Here we show that it is possible to distribute quantum entanglement encoded in OAM over a turbulent intracity link of 3 km. We confirm quantum entanglement of the first two higher-order levels (with OAM=± 1ħ and ± 2ħ). They correspond to four additional quantum channels orthogonal to all that have been used in long-distance quantum experiments so far. Therefore, a promising application would be quantum communication with a large alphabet. We also demonstrate that our link allows access to up to 11 quantum channels of OAM. The restrictive factors toward higher numbers are technical limitations that can be circumvented with readily available technologies.
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, A-1090 Vienna, Austria; Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, A-1090 Vienna, Austria mario.krenn@univie.ac.at anton.zeilinger@univie.ac.at.
authors.forename: Mario
authors.initials: M
authors.lastname: Krenn
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, A-1090 Vienna, Austria; Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, A-1090 Vienna, Austria.
authors.forename: Johannes
authors.initials: J
authors.lastname: Handsteiner
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, A-1090 Vienna, Austria; Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, A-1090 Vienna, Austria.
authors.forename: Matthias
authors.initials: M
authors.lastname: Fink
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, A-1090 Vienna, Austria; Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, A-1090 Vienna, Austria.
authors.forename: Robert
authors.initials: R
authors.lastname: Fickler
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, A-1090 Vienna, Austria; Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, A-1090 Vienna, Austria mario.krenn@univie.ac.at anton.zeilinger@univie.ac.at.
authors.forename: Anton
authors.initials: A
authors.lastname: Zeilinger
keywords: large Hilbert space; photonic orbital angular momentum; photonic spatial modes; quantum communication; quantum entanglement