Figure 7.

Multi-photon quantum experiments involving OAM. In (a), an experiment is shown where two properties of a photon (the polarization and the parity of an ℓ = ±1 spatial mode) are teleported simultaneously. It is possible by a quantum non-demolition measurement, which curiously is implemented itself as a quantum teleportation scheme. For this, six photons are required to teleport the (2 × 2)-dimensional quantum state (adapted from [118]). In (b), an experiment is shown which creates a genuine multi-partite high-dimensional entangled state. Similarly to multi-photon polarization experiments, the which-crystal information is erased by an interferometer that sorts even and odd OAM modes. The resulting state has an asymmetric entanglement structure—a feature that can only exist when both the number of particles and the number of dimensions are larger than 2 (adapted from [75]). (Online version in colour.)