  Photonics has become a mature field of quantum information science, where
integrated optical circuits offer a way to scale the complexity of the setup as
well as the dimensionality of the quantum state. On photonic chips, paths are
the natural way to encode information. To distribute those high-dimensional
quantum states over large distances, transverse spatial modes, like orbital
angular momentum (OAM) possessing Laguerre Gauss modes, are favourable as
flying information carriers. Here we demonstrate a quantum interface between
these two vibrant photonic fields. We create three-dimensional path
entanglement between two photons in a non-linear crystal and use a mode sorter
as the quantum interface to transfer the entanglement to the OAM degree of
freedom. Thus our results show a novel, flexible way to create high-dimensional
spatial mode entanglement. Moreover, they pave the way to implement broad
complex quantum networks where high-dimensionally entangled states could be
distributed over distant photonic chips.
