Observation of Four-Photon Orbital Angular Momentum Entanglement.

@article{Hiesmayr2015ObservationOF,
  title={Observation of Four-Photon Orbital Angular Momentum Entanglement.},
  author={Beatrix C. Hiesmayr and M. J. A. de Dood and Wolfgang L{\"o}ffler},
  journal={Physical review letters},
  year={2015},
  volume={116 7},
  pages={
          073601
        }
}
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… 

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