title: Observation of Four-Photon Orbital Angular Momentum Entanglement.
abstract: We demonstrate genuine multipartite quantum entanglement of four photons in their orbital angular momentum degrees of freedom, where a high-dimensional discrete Hilbert space is attached to each photon. This can encode more quantum information compared to the qubit case, but it is a long-standing problem to entangle more than two such photons. In our experiment we use pulsed spontaneous parametric down-conversion to produce the photon quadruplets, which allows us to detect about one four-photon event per second. By means of quantum state reconstruction and a suitable witness operator we find that the photon quadruplets form a genuine multipartite entangled symmetric Dicke state. This opens a new tool for addressing foundational questions in quantum mechanics, and for exploration of novel high-dimensional multiparty quantum information applications such as secret sharing.
authors.affiliation: Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria.
authors.forename: B C
authors.initials: BC
authors.lastname: Hiesmayr
authors.affiliation: Huygens-Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands.
authors.forename: M J A
authors.initials: MJ
authors.lastname: de Dood
authors.affiliation: Huygens-Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands.
authors.forename: W
authors.initials: W
authors.lastname: Löffler