  Quantum networks are the ultimate target in quantum communication, where many
connected users can share information carried by quantum systems. The keystones
of such structures are the reliable generation, transmission and manipulation
of quantum states. Two-dimensional quantum states, qubits, are steadily adopted
as information units. However, high-dimensional quantum states, qudits,
constitute a richer resource for future quantum networks, exceeding the
limitations imposed by the ubiquitous qubits. The generation and manipulation
of such $D$-level systems have been improved over the last ten years, but their
reliable transmission between remote locations remains the main challenge.
Here, we show how a recent air-core fiber supporting orbital angular momentum
(OAM) modes can be exploited to faithfully transmit $D$-dimensional states.
Four OAM quantum states and their superpositions are created, propagated in a
1.2 km long fiber and detected with high fidelities. In addition, three quantum
key distribution (QKD) protocols are implemented as concrete applications to
assert the practicality of our results. This experiment enhances the
distribution of high-dimensional quantum states, attesting the orbital angular
momentum as vessel for the future quantum network.
