  Versatile and high-brightness sources of high-dimensional entangled photon
pairs are important for emerging quantum technologies such as secure quantum
communication. Here, we experimentally demonstrate a new scalable method to
create photon pairs carrying orbital angular momentum that are entangled in
arbitrarily high dimensions. Our method relies on indistinguishable photon
pairs created coherently in different sources. We demonstrate the creation of
three-dimensionally entangled states and show how to incrementally increase the
dimensionality of entanglement. The generated states retain their quality even
in higher dimensions. In addition, the modular structure of our approach allows
for generalization to various degrees of freedom and even implementation in
integrated compact devices. We therefore expect that future quantum
technologies and fundamental tests of nature in higher dimensions will benefit
from this novel approach.
